Method for manufacturing sputtering target, method for forming oxide film, and transistor

ABSTRACT

A method for manufacturing a sputtering target with which an oxide semiconductor film with a small amount of defects can be formed is provided. Alternatively, an oxide semiconductor film with a small amount of defects is formed. A method for manufacturing a sputtering target is provided, which includes the steps of: forming a polycrystalline In-M-Zn oxide (M represents a metal chosen among aluminum, titanium, gallium, yttrium, zirconium, lanthanum, cesium, neodymium, and hafnium) powder by mixing, sintering, and grinding indium oxide, an oxide of the metal, and zinc oxide; forming a mixture by mixing the polycrystalline In-M-Zn oxide powder and a zinc oxide powder; forming a compact by compacting the mixture; and sintering the compact.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a sputteringtarget, a method for forming an oxide film, and a transistor.

2. Description of the Related Art

Transistors used for most flat panel displays typified by a liquidcrystal display device or a light-emitting display device are formedusing a silicon semiconductor such as amorphous silicon, single crystalsilicon, or polycrystalline silicon provided over a glass substrate.Further, such a transistor employing such a silicon semiconductor isused in integrated circuits (ICs) and the like.

In recent years, attention has been drawn to a technique in which,instead of a silicon semiconductor, a metal oxide exhibitingsemiconductor characteristics is used in transistors. Note that in thisspecification, a metal oxide exhibiting semiconductor characteristics isreferred to as an oxide semiconductor.

For example, InGaO₃(ZnO)_(m) (m: a natural number) having a homologousphase is known as an oxide semiconductor (see Non-Patent Documents 1 and2).

In addition, Patent Document 1 discloses a transparent thin filmfield-effect transistor including a homologous compound InMO₃(ZnO)_(m)(M represents In, Fe, Ga, or Al, and m is an integer greater than orequal to 1 and less than 50).

Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2004-103957

Non-Patent Documents

-   [Non-Patent Document 1] M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J.    Solid State Chem., 1991, Vol. 93, pp. 298-315-   [Non-Patent Document 2] M. Nakamura, N. Kimizuka, T. Mohri, and M.    Isobe, “Syntheses and crystal structures of new homologous    compounds, indium iron zinc oxides (InFeO₃(ZnO)_(m)) (m: natural    number) and related compounds”, KOTAI BUTSURI (SOLID STATE PHYSICS),    1993, Vol. 28, No. 5, pp. 317-327

SUMMARY OF THE INVENTION

An oxide semiconductor film with low crystallinity is likely to includedefects such as oxygen vacancies or dangling bonds.

In the case where stacked oxide semiconductor films formed usingsputtering targets with different compositions have differentcrystallinities, defects are generated at the interface between thestacked oxide semiconductor films.

Defects included in an oxide semiconductor film or defects combined withhydrogen or the like might cause carrier generation and changeelectrical characteristics of the oxide semiconductor film. This resultsin poor electrical characteristics of a transistor and causes anincrease in the amount of change in electrical characteristics of thetransistor, typically the threshold voltage, due to a change over timeor a stress test (e.g., a bias-temperature (BT) stress test or a BTphotostress test).

Therefore, it is an object of one embodiment of the present invention toprovide a method for manufacturing a sputtering target with which anoxide semiconductor film with a small amount of defects can be formed.It is another object of one embodiment of the present invention to forman oxide semiconductor film with a small amount of defects. Anotherobject of one embodiment of the present invention is to improveelectrical characteristics of a semiconductor device or the likeincluding an oxide semiconductor film. Another object of one embodimentof the present invention is to improve reliability of a semiconductordevice including an oxide semiconductor film. Note that in oneembodiment of the present invention, there is no need to achieve all theobjects.

One embodiment of the present invention is a method for manufacturing asputtering target, which includes the steps of: forming apolycrystalline In-M-Zn oxide (M represents aluminum, titanium, gallium,yttrium, zirconium, lanthanum, cesium, neodymium, or hafnium) powder bymixing, sintering, and grinding indium oxide, a metal oxide (the metalis aluminum, titanium, gallium, yttrium, zirconium, lanthanum, cesium,neodymium, or hafnium), and zinc oxide; forming a mixture by mixing thepolycrystalline In-M-Zn oxide powder and a zinc oxide powder; forming acompact by compacting the mixture; and sintering the compact.

Note that the atomic ratio of zinc in the sputtering target is higherthan that of M (M represents aluminum, titanium, gallium, yttrium,zirconium, lanthanum, cesium, neodymium, or hafnium).

The polycrystalline In-M-Zn oxide powder used to manufacture asputtering target is a homologous compound.

One embodiment of the present invention is a method for forming an oxidefilm including an In-M-Zn oxide (M represents aluminum, titanium,gallium, yttrium, zirconium, lanthanum, cesium, neodymium, or hafnium)by a sputtering method using a sputtering target containing indium, M,and zinc, and having an atomic ratio of zinc higher than that of M.

Note that the In-M-Zn oxide (M represents aluminum, titanium, gallium,yttrium, zirconium, lanthanum, cesium, neodymium, or hafnium) has ahomologous structure.

The atomic ratio of Zn to M in the In-M-Zn oxide (M represents aluminum,titanium, gallium, yttrium, zirconium, lanthanum, cesium, neodymium, orhafnium) is higher than 0.5.

One embodiment of the present invention is a transistor containing theIn-M-Zn oxide (M represents aluminum, titanium, gallium, yttrium,zirconium, lanthanum, cesium, neodymium, or hafnium).

In accordance with one embodiment of the present invention, a method formanufacturing a sputtering target with which an oxide semiconductor filmwith a small amount of defects can be formed can be provided. Inaccordance with one embodiment of the present invention, an oxidesemiconductor film with a small amount of defects can be formed. Inaccordance with one embodiment of the present invention, electricalcharacteristics of a semiconductor device or the like including an oxidesemiconductor film can be improved. In accordance with one embodiment ofthe present invention, reliability of a semiconductor device includingan oxide semiconductor film can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C each illustrate a crystal structure of a homologouscompound.

FIG. 2 illustrates a process for manufacturing a sputtering target.

FIG. 3 illustrates a process for manufacturing a sputtering target.

FIGS. 4A to 4C are schematic diagrams illustrating a method formanufacturing an oxide.

FIGS. 5A to 5F are schematic diagrams illustrating oxides.

FIGS. 6A to 6C are schematic diagrams each illustrating a sputteredparticle.

FIGS. 7A and 7B illustrate a crystal structure of a homologous compound.

FIGS. 8A to 8C are schematic diagrams illustrating a method formanufacturing an oxide.

FIGS. 9A to 9C are schematic diagrams each illustrating an oxide.

FIGS. 10A and 10B show nanobeam electron diffraction patterns of CAAC-OSand nc-OS.

FIG. 11 illustrates a process of crystal growth of zinc oxide.

FIGS. 12A to 12C illustrate a process of crystal growth of zinc oxide.

FIGS. 13A and 13B illustrate a process of crystal growth of zinc oxide.

FIGS. 14A to 14C are schematic diagrams illustrating a method formanufacturing an oxide.

FIG. 15 is a schematic diagram illustrating an oxide.

FIGS. 16A to 16C are schematic diagrams illustrating a method formanufacturing an oxide.

FIGS. 17A to 17C are schematic diagrams illustrating a method formanufacturing an oxide.

FIGS. 18A and 18B are schematic diagrams illustrating a method formanufacturing an oxide.

FIG. 19 is a top view of a deposition apparatus.

FIG. 20 is a cross-sectional view of a deposition apparatus.

FIGS. 21A1, 21A2, 21B1, and 21B2 illustrate plasma discharge in asputtering method using a DC power source or an AC power source.

FIGS. 22A to 22C are a top view and cross-sectional views illustratingone embodiment of a transistor.

FIGS. 23A to 23D are cross-sectional views illustrating one embodimentof a transistor.

FIG. 24 is a cross-sectional view illustrating one embodiment of atransistor.

FIGS. 25A to 25D are a top view and cross-sectional views illustratingembodiments of transistors.

FIG. 26 is a cross-sectional view illustrating one embodiment of atransistor.

FIGS. 27A to 27C are a top view and cross-sectional views illustratingone embodiment of a transistor.

FIGS. 28A to 28C are a top view and cross-sectional views illustratingone embodiment of a transistor.

FIGS. 29A to 29C are a top view and cross-sectional views illustratingone embodiment of a transistor.

FIGS. 30A to 30D are cross-sectional views illustrating one embodimentof a method for manufacturing a transistor.

FIGS. 31A to 31C are cross-sectionals views illustrating one embodimentof a method for manufacturing a transistor.

FIG. 32 shows atomic ratios of In—Ga—Zn oxide films which are found fromXPS analysis results.

FIGS. 33A and 33B show results of X-ray diffraction measurement of anIn—Ga—Zn oxide film.

FIGS. 34A and 34B show results of X-ray diffraction measurement of anIn—Ga—Zn oxide film.

FIGS. 35A and 35B show results of X-ray diffraction measurement of anIn—Ga—Zn oxide film.

FIG. 36 is a ternary phase diagram of sputtering targets and In—Ga—Znoxide films.

FIGS. 37A to 37D are cross-sectional TEM images of samples 2 and 3.

FIGS. 38A and 38B are cross-sectional HAADF-STEM images.

FIG. 39 shows band diagrams of In—Ga—Zn oxide films.

FIGS. 40A and 40B show band diagrams of In—Ga—Zn oxide films.

FIG. 41 shows cross-sectional TEM images of samples 11 to 13.

FIG. 42 shows cross-sectional TEM images of samples 11 to 13.

FIG. 43 shows cross-sectional TEM images of samples 12 and 13.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to drawings. Note that the present invention is notlimited to the following description, and it is easily understood bythose skilled in the art that various changes and modifications can bemade without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments andexamples. In addition, in the following embodiments and examples, thesame portions or portions having similar functions are denoted by thesame reference numerals or the same hatching patterns in differentdrawings, and description thereof will not be repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments and examples of thepresent invention are not necessarily limited to such scales.

Furthermore, terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Functions of a “source” and a “drain” are sometimes interchanged witheach other when the direction of current flowing is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

A voltage refers to a difference between potentials of two points, and apotential refers to electrostatic energy (electric potential energy) ofa unit charge at a given point in an electrostatic field. Note that ingeneral, a difference between a potential of one point and a referencepotential (e.g., a ground potential) is simply called a potential or avoltage, and a potential and a voltage are used as synonymous words inmany cases. Thus, in this specification, a potential may be rephrased asa voltage and a voltage may be rephrased as a potential unless otherwisespecified.

In this specification, in the case where an etching step is performedafter a photolithography process, a mask formed in the photolithographyprocess is removed after the etching step.

Embodiment 1

In this embodiment, a method for manufacturing a sputtering target willbe described.

<Homologous Compound and Homologous Structure>

First, homologous compounds represented by InMO₃(ZnO)_(m) (M representsAl, Ti, Ga, Y, Zr, La, Cs, Nd, or Hf, and m is a natural number) aredescribed. The homologous compounds represented by InMO₃(ZnO)_(m) havethe same crystal structure as LuFeO₃(ZnO)_(m) having a layered structurein which LuO₂ ⁻ layers and (FeZn_(m))O_(m+1) ¹⁺ layers are stackedregularly and alternately, with the space group R−3m when m is an oddnumber and with the space group P6₃/mmc when m is an even number. Notethat InMo₃(ZnO)_(m) (m=1) is also referred to as a YbFe₂O₄ structure.Such a crystal structure is referred to as a homologous structure. Notethat in this specification, crystal structures are basically inhexagonal representation.

Next, crystal structures of homologous compounds are described usingInGaO₃(ZnO)_(m) as a typical example of InMO₃(ZnO)_(m). FIGS. 1A to 1Cillustrate crystal structures of InGaO₃(ZnO)_(m) where m=1 (i.e.,InGaO₃(ZnO)₁), where m=2 (i.e., InGaO₃(ZnO)₂), and where m=3 (i.e.,InGaO₃(ZnO)₃), respectively.

As illustrated in FIGS. 1A to 1C, homologous compounds represented byInMO₃(ZnO)_(m) have crystal structures in which pluralities ofrespective repeating units u1 to u3 each including a plurality of layersare stacked. In addition, in each crystal structure, there are (m+1)layers including gallium atoms and/or zinc atoms and oxygen atoms (ZnO,GaO, or (Ga,Zn)O) between adjacent InO₂ layers. Such a structure isreferred to as a homologous structure. Note that a compound other thanthe homologous compounds represented by InMO₃(ZnO)_(m) can have ahomologous structure.

<CAAC-OS Film>

Next, a c-axis aligned crystalline oxide semiconductor (CAAC-OS) will bedescribed. Note that only the crystal structure of the CAAC-OS will bedescribed here, and details of CAAC-OS will be described in Embodiment2.

The CAAC-OS refers to an oxide semiconductor including a c-axis alignedcrystal (CAAC).

For example, the CAAC-OS includes a plurality of crystal parts. In theplurality of crystal parts, c-axes are aligned in a direction parallelto a normal vector of a surface where the CAAC-OS is formed or a topsurface of the CAAC-OS in some cases. When the CAAC-OS is analyzed by anout-of-plane method with an X-ray diffraction (XRD) apparatus, a peakattributable to c-axis alignment, e.g., a peak attributable to the (00x)plane orientation, appears in some cases. Further, for example, spots(bright spots) are shown in an electron diffraction pattern of theCAAC-OS. In the CAAC-OS, for example, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part aredifferent from those of another crystal part, in some cases.

According to a TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface where theCAAC-OS film is formed (hereinafter, a surface where the CAAC-OS film isformed is also referred to as a formation surface) or a top surface ofthe CAAC-OS film, and is arranged in parallel to the formation surfaceor the top surface of the CAAC-OS film.

On the other hand, according to a TEM image of the CAAC-OS film observedin a direction substantially perpendicular to the sample surface (planTEM image), metal atoms are arranged in a triangular or hexagonalconfiguration in the crystal parts. However, there is no regularity ofarrangement of metal atoms between different crystal parts.

In this specification, a simple term “perpendicular” includes a rangefrom 80° to 100° or from 85° to 95°. In addition, a simple term“parallel” includes a range from −10° to 10° or from −5° to 5°.

The CAAC-OS has a homologous structure when having at least one layerincluding gallium atoms and/or zinc atoms and oxygen atoms (ZnO, GaO, or(Ga,Zn)O) between adjacent InO₂ layers.

<First Method for Manufacturing Sputtering Target>

Next, a method for manufacturing a sputtering target with which anIn—Ga—Zn oxide film having a homologous structure can be formed will bedescribed with reference to FIG. 2. In addition, a method formanufacturing a sputtering target with which a CAAC-OS film having ahomologous structure can be formed will be described with reference toFIG. 2.

As illustrated in FIG. 2, in a step S101, a mixture is formed bypreparing, grinding, and mixing an indium oxide powder, a gallium oxidepowder, and a zinc oxide powder that are materials of an In—Ga—Zn oxide.Powders with purities of 99.9% or higher, 99.99% or higher, or 99.999%or higher are used as the indium oxide, gallium oxide, and zinc oxidepowders. Accordingly, the concentration of impurities contained in anoxide semiconductor film formed later can be reduced, and a transistorwith excellent electrical characteristics can be manufactured.

Alternatively, a polycrystalline In—Ga—Zn oxide powder manufacturedthrough steps S111 to S113 in FIG. 3 which will be described later and azinc oxide powder can be used as materials of the In—Ga—Zn oxide.Alternatively, the polycrystalline In—Ga—Zn oxide powder manufacturedthrough the steps S111 to S113 in FIG. 3 and a Ga—Zn oxide powder can beused as materials of the In—Ga—Zn oxide.

The materials of the In—Ga—Zn oxide are prepared such that the atomicratio of Zn in the mixture is higher than that of Ga in the mixture. Forexample, the materials of the In—Ga—Zn oxide powder are prepared atIn:Ga:Zn=1:3:4, In:Ga:Zn=1:3:5, In:Ga:Zn=1:3:6, In:Ga:Zn=1:3:7,In:Ga:Zn=1:3:8, In:Ga:Zn=1:3:9, In:Ga:Zn=1:3:10, In:Ga:Zn=1:4:5,In:Ga:Zn=1:4:6, In:Ga:Zn=1:4:7, In:Ga:Zn=1:4:8, In:Ga:Zn=1:4:9,In:Ga:Zn=1:4:10, In:Ga:Zn=1:5:6, In:Ga:Zn=1:5:7, In:Ga:Zn=1:5:8,In:Ga:Zn=1:5:9, In:Ga:Zn=1:5:10, In:Ga:Zn=1:6:7, In:Ga:Zn=1:6:8,In:Ga:Zn=1:6:9, or In:Ga:Zn=1:6:10. Accordingly, a sputtering targetcontaining an In—Ga—Zn oxide having a homologous structure can bemanufactured in a later sintering step.

The materials of the In—Ga—Zn oxide can be ground and mixed using amixing grinder such as a ball mill, a bead mill, a roll mill, a jetmill, or an ultrasonic device. The use of the mixing grinder enables thematerials of the In—Ga—Zn oxide to be ground into particles of apredetermined size and mixed.

The particles of the ground materials of the In—Ga—Zn oxide preferablyhave an average size of greater than or equal to 0.01 μm and less thanor equal to 3.0 μm, or greater than or equal to 0.1 μm and less than orequal to 2.0 μm.

Next, in a step S102, the mixture is compacted to form a compact.

Examples of methods for forming the compact include metal molding, coldisostatic pressing, and the like. Note that in the compacting process, acompacting aid such as polyvinyl alcohol, methyl cellulose, polywax, oran oleic acid may be used as appropriate.

In the step S101, a slurry may be formed by mixing the materials of theIn—Ga—Zn oxide with water, a dispersant, and a binder, and in the stepS102, the compact may be formed by pouring the slurry into a mold,suctioning water from the bottom of the mold, and performing dryingtreatment. In the drying treatment, moisture contained in the compactcan be removed by performing heat treatment at 300° C. to 700° C. afternatural drying.

Next, in a step S103, the compact is sintered to form a sinteredcompact.

In the sintering step in the step S103, the compact is heated at 1200°C. to 1600° C., or 1300° C. to 1500° C. By this step, a polycrystallineIn—Ga—Zn oxide can be formed as the sintered compact. In thepolycrystalline In—Ga—Zn oxide, the atomic ratio of Zn is higher thanthat of Ga.

Then, the sintered compact may be subjected to heat treatment in areducing atmosphere of hydrogen, methane, carbon monoxide, or the likeor in an inert gas atmosphere of nitrogen, a rare gas, or the like.Accordingly, resistance variation of the sintered compact can bereduced.

Note that a sputtering target can be manufactured by performing the stepS102 (the compacting step) and the step S103 (the sintering step) at thesame time. Examples of such compacting methods include hot pressing, hotisostatic pressing, and the like.

Next, in a step S104, the sintered compact is processed to manufacture asputtering target.

In the step S104, the sintered compact is processed by cutting and isthen mounted on a mounting jig such as a backing plate. After thecutting, the sintered compact is subjected to mirror finishing to asurface roughness (Ra) of 5 μm or less, or 2 μm or less. Examples ofmirror finishing methods include mechanical polishing, chemicalpolishing, CMP, and the like.

Through the above steps, the sputtering target can be manufactured. Inthe sputtering target manufactured in this embodiment, the atomic ratioof Zn is higher than that of Ga. By a sputtering method using such asputtering target, a film of an In—Ga—Zn oxide having a homologousstructure can be formed. Furthermore, a film of an In—Ga—Zn oxide thathas a homologous structure and is CAAC-OS can be formed.

<Second Method for Manufacturing Sputtering Target>

Here, a method for manufacturing a sputtering target with which a filmof a homologous compound represented by InGaO₃(ZnO)_(m) can be formedwill be described with reference to FIG. 3. In addition, a method formanufacturing a sputtering target with which a film of CAAC-OS that is ahomologous compound represented by InGaO₃(ZnO)_(m) can be formed will bedescribed with reference to FIG. 3.

As illustrated in FIG. 3, an In—Ga—Zn oxide powder is manufacturedfirst.

In a step S111, a mixture is formed by preparing and mixing appropriateamounts of an indium oxide powder, a gallium oxide powder, and a zincoxide powder that are materials of an In—Ga—Zn oxide. Powders withpurities of 99.9% or higher, 99.99% or higher, or 99.999% or higher areused as the indium oxide, gallium oxide, and zinc oxide powders.Accordingly, the concentration of impurities contained in an oxidesemiconductor film formed later can be reduced, and a transistor withexcellent electrical characteristics can be manufactured.

The amounts of the indium oxide powder, the gallium oxide powder, andthe zinc oxide powder are preferably adjusted such that the mixtureincludes metal elements at an atomic ratio of In:Ga:Zn=1:1:m (m is anatural number). Accordingly, a sputtering target containing an In—Ga—Znoxide having a homologous structure and represented by InGaO₃(ZnO)_(m)can be manufactured in a sintering step in a step S112.

Next, in the step S112, the mixture is sintered to form apolycrystalline In—Ga—Zn oxide.

The sintering step can be performed in a manner similar to that of thesintering step in the step S103 in FIG. 2. By this step, thepolycrystalline In—Ga—Zn oxide can be formed. The polycrystallineIn—Ga—Zn oxide is a homologous compound represented by InGaO₃(ZnO)_(m),typical examples of which include InGaZnO₄, InGaZn₂O₅, InGaZn₃O₆,InGaZn₄O₇, InGaZn₅O₈, and the like.

Next, in the step S113, the polycrystalline In—Ga—Zn oxide is groundinto a polycrystalline In—Ga—Zn oxide powder.

The polycrystalline In—Ga—Zn oxide can be ground using a grinding meanssuch as a ball mill, a bead mill, a roll mill, a jet mill, or anultrasonic device.

The ground polycrystalline In—Ga—Zn oxide powder preferably has anaverage particle size of greater than or equal to 0.01 μm and less thanor equal to 3.0 μm, or greater than or equal to 0.1 μm and less than orequal to 2.0 μm.

Note that the grinding step in the step S113 is preferably performedagain in the case where the average particle size of the groundpolycrystalline In—Ga—Zn oxide powder is 3.0 μm or larger.

Next, a process for manufacturing a sputtering target will be described.

In a step S114, the polycrystalline In—Ga—Zn oxide powder and a zincoxide powder are mixed to form a mixture. Note that in this mixing step,the polycrystalline In—Ga—Zn oxide powder and the zinc oxide powder mayeach be ground in order to improve the uniformity of powder particlesize.

Here, the polycrystalline In—Ga—Zn oxide powder that is a homologouscompound and obtained in the step S113 is mixed with a zinc oxide powderto form a mixture. The polycrystalline In—Ga—Zn oxide powder and thezinc oxide powder are adjusted such that the atomic ratio of Zn to Ga inthe mixture is higher than or equal to (m+0.05) and lower than or equalto (m+0.5). For example, the polycrystalline In—Ga—Zn oxide powder andthe zinc oxide powder are mixed at In:Ga:Zn=1:1:1.05, In:Ga:Zn=1:1:2.05,or In:Ga:Zn=1:1:3.05. Accordingly, a sputtering target of a homologouscompound represented by InGaO₃(ZnO)_(m) in which the number of atoms ofzinc is larger than that of gallium can be manufactured in a latersintering step in a step S116.

Next, in a step S115, the mixture is compacted to form a compact.

The compact can be formed by compacting the mixture in a manner similarto that of the compacting step in the step S102 in FIG. 2.

In the step S114, a slurry may be formed by mixing the polycrystallineIn—Ga—Zn oxide powder and the zinc oxide powder with water, adispersant, and a binder, and in the step S115, the compact may beformed by pouring the slurry into a mold, suctioning water from thebottom of the mold, and performing drying treatment. In the dryingtreatment, moisture contained in the compact can be removed byperforming heat treatment at 300° C. to 700° C. after natural drying.

Next, in the step S116, the compact is sintered to form a sinteredcompact.

In the sintering step in the step S116, the compact is heated at 800° C.to 1600° C., or 1300° C. to 1500° C. By this step, a polycrystallineIn—Ga—Zn oxide can be formed as the sintered compact. In thepolycrystalline In—Ga—Zn oxide, the atomic ratio of Zn is higher thanthat of Ga. The polycrystalline In—Ga—Zn oxide is a mixed crystal of ZnOand a homologous compound represented by InGaO₃(ZnO)_(m).

Then, the sintered compact may be subjected to heat treatment in areducing atmosphere of hydrogen, methane, carbon monoxide, or the likeor in an inert gas atmosphere of nitrogen, a rare gas, or the like.Accordingly, resistance variation of the sintered compact can bereduced.

Note that the sintered compact can be formed by performing the step S115(the compacting step) and the step S116 (the sintering step) at the sametime. Examples of such compacting methods include hot pressing, hotisostatic pressing, and the like.

Next, in a step S117, the sintered compact is processed to manufacture asputtering target.

In the step S117, the processing step in the step S104 in FIG. 2 can beemployed as appropriate.

Through the above steps, the sputtering target can be manufactured. Thesputtering target manufactured in this embodiment is a mixed crystal ofZnO and a homologous compound represented by InGaO₃(ZnO)_(m), and theatomic ratio of Zn to Ga in the sputtering target is higher than orequal to (m+0.05) and lower than or equal to (m+0.5). By a sputteringmethod using such a sputtering target, a film of an In—Ga—Zn oxide thatis a homologous compound can be formed. Furthermore, a film of anIn—Ga—Zn oxide that is a homologous compound and is CAAC-OS can beformed.

Embodiment 2

A method for forming an oxide film using the sputtering targetmanufactured in Embodiment 1 will be described in this embodiment withreference to FIGS. 4A to 4C, FIGS. 5A to 5F, FIGS. 6A to 6C, FIGS. 7Aand 7B, FIGS. 8A to 8C, FIGS. 9A to 9C, FIGS. 10A and 10B, FIG. 11,FIGS. 12A to 12C, FIGS. 13A and 13B, FIGS. 14A to 14C, FIG. 15, FIGS.16A to 16C, FIGS. 17A to 17C, and FIGS. 18A and 18B. Here, a descriptionis given using an In—Ga—Zn oxide as a typical example of an In-M-Znoxide (M represents Al, Ti, Ga, Y, Zr, La, Cs, Nd, or Hf).

FIGS. 4A to 4C, FIGS. 8A to 8C, FIGS. 14A to 14C, FIGS. 16A to 16C,FIGS. 17A to 17C, and FIGS. 18A and 18B are schematic diagramsillustrating a sputtering process in a deposition chamber of adeposition apparatus. Note that the deposition chamber of the depositionapparatus will be briefly described in this embodiment and will bedescribed in detail in Embodiment 3.

<Heating Deposition (Deposition Temperature: Higher than or Equal to150° C. and Lower than 600° C.>

As illustrated in FIG. 4A, a substrate stage 12 and a sputtering target13 manufactured in Embodiment 1 are provided so as to face each other ina deposition chamber 11 of a deposition apparatus. The substrate stage12 is provided with a substrate 121.

A sputtering gas such as oxygen or an inert gas like argon is introducedinto the deposition chamber 11 that is under reduced pressure, and avoltage is applied to the sputtering target 13 to generate plasma 17.The sputtering gas is ionized in the plasma 17, and ions 15 aregenerated. When the ions 15 collide with the sputtering target 13,interatomic bonds in the sputtering target 13 are cut and particles areseparated from the sputtering target 13. Therefore, ions, sputteredparticles, electrons, and/or the like exist in the plasma 17. Here,particles separated from a sputtering target are referred to assputtered particles.

An example of the ions 15 is oxygen cations. With use of oxygen cationsas the ions 15, plasma damage during deposition, for example, can bereduced. In addition, with use of oxygen cations as the ions 15, thesputtering target 13 can be prevented from decreasing its crystallinityor becoming amorphous by collision of the ions 15 with a surface of thesputtering target 13, for example. Furthermore, with use of oxygencations as the ions 15, the crystallinity of the sputtering target 13may be increased by collision of the ions 15 with a surface of thesputtering target 13, for example. Note that cations of a rare gas (suchas helium, neon, argon, krypton, or xenon), for example, may be used asthe ions 15.

Examples of the sputtered particles include zinc particles, oxygenparticles, zinc oxide particles, In—Ga—Zn oxide particles, and the like.The sputtering target manufactured in Embodiment 1 contains Zn at ahigher ratio than that of Ga. For this reason, a description is givenhere using a model in which zinc particles, oxygen particles, or zincoxide particles are preferentially separated from the sputtering target13 and then zinc particles, oxygen particles, zinc oxide particles, andIn—Ga—Zn oxide particles are separated.

First, zinc particles 123 a and oxygen particles 123 b are separated assputtered particles from the sputtering target 13. Next, the zincparticles 123 a and the oxygen particles 123 b move to the substrate121, whereby a hexagonal crystal grain 123 c of zinc oxide is formedover the substrate.

FIG. 5A illustrates a model of a top-view shape of the hexagonal crystalgrain 123 c of zinc oxide. As illustrated in FIG. 5A, Zn atoms and Oatoms are bound in a hexagonal shape in the hexagonal crystal grain 123c of zinc oxide.

As will be described later in the section “Mechanism of Zinc OxideCrystal Growth,” the crystal of zinc oxide grows rapidly in a directionparallel to the a-b plane. Therefore, the hexagonal crystal grain 123 cof zinc oxide grows in a direction parallel to a surface of thesubstrate 121, that is, in a lateral direction in a cross section of azinc oxide film, at a substrate temperature of higher than or equal to150° C. and lower than 600° C. As a result, a hexagonal-crystal zincoxide film 125 is formed as illustrated in FIG. 4B. That is, thehexagonal-crystal zinc oxide film 125 includes a single crystal region.Note that the hexagonal-crystal zinc oxide film 125 may include anon-single-crystal region.

FIG. 5B illustrates a model of a top-view shape of a region 126 of thehexagonal-crystal zinc oxide film 125, and FIG. 5C illustrates a modelof a cross-sectional shape of the region 126. As illustrated in FIG. 5B,Zn atoms and O atoms are bound in a hexagonal shape in thehexagonal-crystal zinc oxide film 125. The binding of Zn atoms and Oatoms in the hexagonal shape extends over the a-b plane.

Next, sputtered particles are released from the sputtering target. Here,In—Ga—Zn oxide particles are separated as the sputtered particles, andas illustrated in FIG. 4C, In—Ga—Zn oxide particles 127 are depositedover the hexagonal-crystal zinc oxide film 125, whereby a film 129including the In—Ga—Zn oxide particles is formed. Note that zincparticles, oxygen particles, and zinc oxide particles, which are alsoseparated in this step as sputtered particles, are omitted here.

The In—Ga—Zn oxide particles 127 have crystallinity and are typicallysingle crystal. Note that the In—Ga—Zn oxide particles 127 may bepolycrystalline.

Here, the shape of the In—Ga—Zn oxide particle 127 will be describedwith reference to FIGS. 6A to 6C. As illustrated in FIG. 6A, theIn—Ga—Zn oxide particle 127 has a flat-plate-like or flat shape having alength larger than a thickness in a cross section. Note that the lengthin the cross section corresponds to a side parallel to the c-axis of theIn—Ga—Zn oxide particle 127, and the thickness in the cross sectioncorresponds to a side parallel to an axis intersecting the c-axis of theIn—Ga—Zn oxide particle 127. As illustrated in FIG. 6A, the In—Ga—Znoxide particle 127 preferably has two parallel planes 127 s in the formof a regular hexagon that is a hexagon whose interior angles are all120°. Alternatively, as illustrated in FIG. 6C, the In—Ga—Zn oxideparticle 127 preferably has two parallel planes 127 s in the form of aregular triangle that is a triangle whose interior angles are all 60°.The In—Ga—Zn oxide particle 127 can be referred to as a pellet. Theplanes 127 s of the pellet are parallel to the a-b plane of a crystal,for example. Furthermore, the planes 127 s of the pellet areperpendicular to the c-axis direction of the crystal, for example. Theplanes of the pellet have a size of greater than or equal to 1 nm andless than or equal to 100 nm, greater than or equal to 1 nm and lessthan or equal to 30 nm, or greater than or equal to 1 nm and less thanor equal to 10 nm, for example.

Note that the In—Ga—Zn oxide particle 127 is positively or negativelycharged. This is because part of oxygen of an In—Ga—Zn oxide particle127 a is charged by collision with ions, or because part of oxygen ofthe In—Ga—Zn oxide particle 127 a is charged by exposure to the plasma.FIG. 6B is a schematic diagram of the In—Ga—Zn oxide particle 127 awhich is negatively charged. As illustrated in FIG. 6B, part of oxygenincluded in the In—Ga—Zn oxide particle 127 a may be negatively charged.Alternatively, oxygen ions may be bound to the In—Ga—Zn oxide particle127 a.

Here, a crystal structure of a homologous compound represented byInGaO₃(ZnO)_(m) (m is a natural number) where m=1, when seen in adirection parallel to the a-b plane is shown as an example of a crystalincluded in a sputtering target (see FIG. 7A). FIG. 7B illustrates anenlarged view of a portion surrounded by a dashed line in FIG. 7A.

For example, in a crystal included in a sputtering target, there may bea cleavage plane between a first layer including gallium atoms and/orzinc atoms and oxygen atoms and a second layer including gallium atomsand/or zinc atoms and oxygen atoms, as illustrated in FIG. 7B. This isbecause oxygen atoms in the first layer and oxygen atoms in the secondlayer are close to each other (see surrounded portions in FIG. 7B). Forexample, since the oxygen atoms have negative charge, the bindingbetween the layers can be weakened by the oxygen atoms close to eachother. In other words, chemical bonds within each of the first andsecond layers become much stronger than chemical bonds between the firstand second layers, and the cleavage plane is formed between the firstand second layers. In this manner, the cleavage plane may be a planeparallel to the a-b plane.

In addition, the crystal structure illustrated in FIGS. 7A and 7B has aregular triangular or regular hexagonal atomic arrangement of metalatoms in the direction perpendicular to the a-b plane. Therefore, in thecase where the sputtering target including the crystal having thecrystal structure illustrated in FIGS. 7A and 7B is used, theprobability of the In—Ga—Zn oxide particle 127 becoming a shape havingregular hexagonal planes with internal angles of 120° or regulartriangular planes with internal angles of 60° is thought to be high.

Here, typical examples of the crystal structure of the In—Ga—Zn oxideparticle 127 formed by separation along the cleavage plane illustratedin FIG. 7B are illustrated in FIGS. 5D and 5E.

An In—Ga—Zn oxide particle 127 a illustrated in FIG. 5D has a structurein which three layers of a first layer including gallium atoms and/orzinc atoms and oxygen atoms (denoted by (Ga,Zn)O), an indium oxide layer(InO₂), and a second layer including gallium atoms and/or zinc atoms andoxygen atoms (denoted by (Ga,Zn)O) are bound in this order.

An In—Ga—Zn oxide particle 127 b illustrated in FIG. 5E has a structurein which five layers of a first layer including gallium atoms and/orzinc atoms and oxygen atoms (denoted by (Ga,Zn)O), a second layerincluding gallium atoms and/or zinc atoms and oxygen atoms (denoted by(Ga,Zn)O), an indium oxide layer (InO₂), a third layer including galliumatoms and/or zinc atoms and oxygen atoms (denoted by (Ga,Zn)O), and afourth layer including gallium atoms and/or zinc atoms and oxygen atoms(denoted by (Ga,Zn)O) are bound in this order.

In the case where the hexagonal-crystal zinc oxide film 125 is formedover the substrate 121, the In—Ga—Zn oxide particle 127 is deposited soas to be aligned with the orientation of the hexagonal-crystal zincoxide film 125. Specifically, since the substrate is heated, theIn—Ga—Zn oxide particle 127 separated from the sputtering target 13 ismoved or rotated by thermal energy in the vicinity of thehexagonal-crystal zinc oxide film 125 such that the c-axis of theIn—Ga—Zn oxide particle 127 is parallel to the c-axis of thehexagonal-crystal zinc oxide film 125, and after that, the In—Ga—Znoxide particle 127 is deposited over the hexagonal-crystal zinc oxidefilm 125.

At this time, the a-b plane of the In—Ga—Zn oxide particle 127 may berotated and bound such that the a-axis and b-axis directions thereof arealigned with those of the In—Ga—Zn oxide particle that has already beendeposited. As a result, the a-axis and b-axis directions are alignedwith those of an adjacent In—Ga—Zn oxide particle, and therefore, asingle crystal region is formed in the film 129 including the In—Ga—Znoxide particles. In other words, there is a case where the crystalorientation of the film 129 including the In—Ga—Zn oxide particles isuniform over the entire area of the hexagonal-crystal zinc oxide film125, and the film 129 including the In—Ga—Zn oxide particles is singlecrystal. Alternatively, there is a case where a plurality of singlecrystal regions are formed in the film 129 including the In—Ga—Zn oxideparticles and the single crystal regions are aligned only in the c-axisdirection and not in the a-axis and b-axis directions.

FIG. 5F illustrates a model of a cross-sectional shape of the vicinityof the interface between the zinc oxide film and the film 129 includingthe In—Ga—Zn oxide particles in the region 128 illustrated in FIG. 4C.As illustrated in FIG. 5F, Zn of the hexagonal-crystal zinc oxide filmis bound to oxygen of a layer including gallium atoms and/or zinc atomsand oxygen atoms (denoted by (Ga,Zn)O) which is included in the film 129including the In—Ga—Zn oxide particles.

Since the hexagonal-crystal zinc oxide film 125 has high crystallinity,the crystallinity of the film 129 including the In—Ga—Zn oxide particlescan be increased using the hexagonal-crystal zinc oxide film 125 as aseed crystal.

Next, as illustrated in FIG. 8A, zinc particles 123 a and oxygenparticles 123 b are separated from the sputtering target and moved tothe In—Ga—Zn oxide particle 127, whereby a hexagonal crystal grain 123 cof zinc oxide is formed over the substrate in a manner similar to thatin FIG. 4A.

Since the crystal of zinc oxide grows rapidly in a direction parallel tothe a-b plane, the hexagonal crystal grain 123 c of zinc oxide grows ina direction parallel to the surface of the substrate 121, that is, alateral direction in a cross section of a zinc oxide film, whereby ahexagonal-crystal zinc oxide film 131 is formed in a manner similar tothat in FIG. 4B (see FIG. 8B). That is, the hexagonal-crystal zinc oxidefilm 131 includes a single crystal region.

After that, sputtered particles are released from the sputtering target,and In—Ga—Zn oxide particles 133 are deposited over thehexagonal-crystal zinc oxide film 131 as illustrated in FIG. 8C, in amanner similar to that in FIG. 4C. In addition, another In—Ga—Zn oxideparticle is deposited over the In—Ga—Zn oxide particles 133.

A highly crystalline oxide film can be formed by repeating the step offorming the hexagonal-crystal zinc oxide film 131 which is illustratedin FIG. 8B and the step of depositing the In—Ga—Zn oxide particles 133which is illustrated in FIG. 8C.

The atomic ratio of Ga to In (Ga/In) and the atomic ratio of Zn to In(Zn/In) in an In—Ga—Zn oxide formed by a sputtering method using thesputtering target of Embodiment 1 are lower than those in the sputteringtarget. The atomic ratio of Zn to Ga (Zn/Ga) in the In—Ga—Zn oxide filmis higher than or equal to 0.5.

Note that since the In—Ga—Zn oxide particles are deposited so as to bealigned with the c-axis direction of the hexagonal-crystal zinc oxidefilm, the oxide film obtained through this process is a CAAC-OS filmwith the c-axis aligned in a direction parallel to a normal vector of aformation surface or a normal vector of a surface of the CAAC-OS film.

A crystal structure of an oxide film obtained through the depositionprocess in FIGS. 4A to 4C and FIGS. 8A to 8C is described with referenceto FIGS. 9A to 9C. The oxide film obtained through the depositionprocess in FIGS. 4A to 4C and FIGS. 8A to 8C has a homologous structurebecause a plurality of In—Ga—Zn oxide particles each having at least onelayer including gallium atoms and/or zinc atoms and oxygen atoms betweentwo indium oxide layers (InO₂) are stacked.

As illustrated in FIG. 9A, the oxide film obtained through thedeposition process in FIGS. 4A to 4C and FIGS. 8A to 8C has a structurein which three layers of a first indium oxide layer (InO₂), a layerincluding gallium atoms and/or zinc atoms and oxygen atoms (denoted by(Ga,Zn)O), and a second indium oxide layer (InO₂) are bound in thisorder. In other words, one layer including gallium atoms and/or zincatoms and oxygen atoms is provided between indium oxide layers.

Alternatively, as illustrated in FIG. 9B, the oxide film obtainedthrough the deposition process in FIGS. 4A to 4C and FIGS. 8A to 8C hasa structure in which four layers of a first indium oxide layer (InO₂), afirst layer including gallium atoms and/or zinc atoms and oxygen atoms(denoted by (Ga,Zn)O), a second layer including gallium atoms and/orzinc atoms and oxygen atoms (denoted by (Ga,Zn)O), and a second indiumoxide layer (InO₂) are bound in this order. In other words, two layersincluding gallium atoms and/or zinc atoms and oxygen atoms are providedbetween indium oxide layers.

Alternatively, as illustrated in FIG. 9C, the oxide film obtainedthrough the deposition process in FIGS. 4A to 4C and FIGS. 8A to 8C hasa structure in which five layers of a first indium oxide layer (InO₂), afirst layer including gallium atoms and/or zinc atoms and oxygen atoms(denoted by (Ga,Zn)O), a zinc oxide layer (ZnO), a second layerincluding gallium atoms and/or zinc atoms and oxygen atoms (denoted by(Ga,Zn)O), and a second indium oxide layer (InO₂) are bound in thisorder. In other words, two layers including gallium atoms and/or zincatoms and oxygen atoms and a zinc oxide layer are provided betweenindium oxide layers.

Note that a repeating unit structure including a zinc oxide layerbetween a plurality of layers including gallium atoms and/or zinc atomsand oxygen atoms (denoted by (Ga,Zn)O) is formed in a region where theIn—Ga—Zn oxide particle 127, the hexagonal-crystal zinc oxide film 131,and the In—Ga—Zn oxide particle 133 are stacked, as illustrated in FIG.8C.

Note that although the step of forming the hexagonal-crystal zinc oxidefilm 131 which is illustrated in FIG. 8B and the step of depositing theIn—Ga—Zn oxide particles 133 which is illustrated in FIG. 8C aredescribed here as different steps, these steps may be performed at thesame time. In that case, in the step of FIG. 8B, the hexagonal-crystalzinc oxide film 131 may be formed on side surfaces as well as topsurfaces of the In—Ga—Zn oxide particles.

Through the above process, the CAAC-OS film can be formed. In addition,an In—Ga—Zn oxide film having a homologous structure can be formed. Notethat with use of Al, Ti, Y, Zr, La, Cs, Nd, or Hf as appropriate insteadof Ga in the In—Ga—Zn oxide, an In-M-Zn oxide (M represents Al, Ti, Y,Zr, La, Cs, Nd, or Hf) can be deposited. Note that instead of the modelin which the crystal grain 123 c is formed over the substrate 121, theCAAC-OS film can be formed according to a model in which the In—Ga—Znoxide particle 127 a illustrated in FIG. 5D or the In—Ga—Zn oxideparticle 127 b illustrated in FIG. 5E is formed over the substrate 121.In addition, a film of an In—Ga—Zn oxide having a homologous structurecan be formed.

Here, details of the CAAC-OS film formed are described.

The CAAC-OS film is one of oxide semiconductor films including aplurality of c-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film,it is difficult to clearly find a boundary between crystal parts, thatis, a grain boundary. Thus, in the CAAC-OS film, a reduction in electronmobility due to the grain boundary is less likely to occur.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm Thus, there is a case where acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconcatenated to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm² or more, 5μm² or more, or 1000 μm² or more is observed in some cases in the planTEM image.

For example, the CAAC-OS includes a plurality of crystal parts. In theplurality of crystal parts, c-axes are aligned in a direction parallelto a normal vector of a surface where the CAAC-OS is formed or a normalvector of a surface of the CAAC-OS in some cases. When the CAAC-OS isanalyzed by an out-of-plane method with an X-ray diffraction (XRD)apparatus, a peak attributable to c-axis alignment, e.g., a peakattributable to the (00x) plane orientation, appears in some cases.

In the CAAC-OS film having c-axis alignment, while the directions ofa-axes and b-axes are different between crystal parts, the c-axes arealigned in a direction parallel to a normal vector of a formationsurface or a normal vector of a top surface. Thus, each metal atom layerarranged in a layered manner observed in the cross-sectional TEM imagecorresponds to a plane parallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface of the CAAC-OS film. Thus, for example, in thecase where a shape of the CAAC-OS film is changed by etching or thelike, the c-axis might not be necessarily parallel to a normal vector ofa formation surface or a normal vector of a top surface of the CAAC-OSfilm.

Further, distribution of c-axis aligned crystal parts in the CAAC-OSfilm is not necessarily uniform. For example, in the case where crystalgrowth leading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in some cases. Further,when an impurity is added to the CAAC-OS film, a region to which theimpurity is added is altered, and the proportion of the c-axis alignedcrystal parts in the CAAC-OS film varies depending on regions, in somecases.

Further, for example, spots (bright spots) are shown in an electrondiffraction pattern of the CAAC-OS. Furthermore, electron diffractionusing an electron beam having a probe diameter (e.g., larger than orequal to 1 nm and smaller than or equal to 30 nm) close to or smallerthan the size of a crystal part is also referred to as a nanobeamelectron diffraction.

FIG. 10A shows an example of a nanobeam electron diffraction pattern ofa sample including CAAC-OS. Here, the sample is cut in the directionperpendicular to a surface where the CAAC-OS is formed and the thicknessthereof is reduced to about 40 nm. Further, an electron beam with adiameter of 1 nmφ enters from the direction perpendicular to the cutsurface of the sample. FIG. 10A shows that spots are observed in thenanobeam electron diffraction pattern of the CAAC-OS.

The CAAC-OS film is an oxide semiconductor film with a low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element, such as silicon,that has higher bonding strength to oxygen than a metal element includedin the oxide semiconductor film disturbs the atomic arrangement of theoxide semiconductor film by depriving the oxide semiconductor film ofoxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand has high reliability. Electric charges trapped by the carrier trapsin the oxide semiconductor film take a long time to be released, andmight behave like fixed electric charges. Thus, the transistor whichincludes the oxide semiconductor film having high impurity concentrationand a high density of defect states has unstable electricalcharacteristics in some cases.

With the use of highly purified intrinsic or substantially highlypurified intrinsic CAAC-OS in a transistor, variation in electricalcharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small.

<Mechanism of Zinc Oxide Crystal Growth>

Here, a mechanism of crystal growth of the zinc oxide film 125illustrated in FIG. 4B is described with reference to FIG. 11, FIGS. 12Ato 12C, and FIGS. 13A and 13B.

Motion of atoms in heat treatment was examined by a classical moleculardynamics method. An empirical potential which characterizes theinteraction between atoms is defined in a classical molecular dynamicsmethod, so that force that acts on each atom can be determined. Here, alaw of classical mechanics was applied to each atom and Newton'sequation of motion was numerically solved, whereby motion(time-dependent change) of each atom was examined. In this calculation,a Born-Mayer-Huggins potential was used as the empirical potential.

A model in which crystal nuclei 180 that are single crystal zinc oxide(hereinafter referred to as c-ZnO) having a width of 1 nm are providedat equal intervals in amorphous zinc oxide (hereinafter referred to asa-ZnO) was formed as illustrated in FIG. 11. Note that the density ofa-ZnO and c-ZnO was 5.5 g/cm³. The vertical direction was a c-axisdirection.

Next, the model in FIG. 11 was subjected to classical molecular dynamicscalculation at 700° C. for 100 psec (in increments of 0.2 fsec in500,000 steps) with fixed c-ZnO under three-dimensional periodicboundary conditions. Results thereof are shown in FIGS. 12A to 12C andFIGS. 13A and 13B.

FIGS. 12A, 12B, and 12C respectively show the changes of atomicpositions after 20 psec, 40 psec, and 60 psec. FIGS. 13A and 13Brespectively show the changes of atomic positions after 80 psec and 100psec. In each drawing, the distance and direction of crystal growth aredenoted by the length and pointing direction of arrows.

Table 1 shows rates of crystal growth in the vertical direction (c-axis[001] direction) and in the horizontal direction perpendicular thereto.

TABLE 1 Rate of crystal growth Direction (nm/psec) vertical 6.1 × 10⁻³horizontal 3.0 × 10⁻²

In FIGS. 12A to 12C, arrows 184 a, 184 b, 188 a, 188 b, 192 a, and 192 bin the horizontal direction (a direction perpendicular to a c-axisdirection) are longer than arrows 182, 186, and 190 in the verticaldirection (the c-axis direction). Therefore, it is found that crystalgrowth in the horizontal direction is preferential and that the crystalgrowth is finished between adjacent crystal nuclei in FIG. 12C.

In FIGS. 13A and 13B, it is found that crystal growth is carried out inthe vertical direction (the c-axis direction) using crystal regionsformed at the surface as seed crystals as indicated by arrows 194 and196.

It is found from Table 1 that the rate of crystal growth in thehorizontal direction is approximately 4.9 times as high as that in thevertical direction (c-axis [001] direction). Accordingly, crystal growthof ZnO first proceeds in a direction parallel to a surface (a-b plane).At this time, crystal growth proceeds in the horizontal direction on thea-b plane, and a single crystal region is formed. Next, crystal growthproceeds in the c-axis direction, i.e., a direction perpendicular to thesurface (a-b plane), using the single crystal region formed at thesurface (a-b plane) as a seed crystal. Therefore, ZnO tends to havec-axis alignment. In this manner, the single crystal region is formed bypreferential crystal growth in the direction parallel to the surface(a-b plane) and then by crystal growth in the c-axis direction, which isthe direction perpendicular to the surface (also referred to asepitaxial growth or axial growth).

<Heating Deposition (Deposition Temperature: Higher than or Equal to600° C. and Lower than Substrate Strain Point)>

Next, a deposition method different from that in FIGS. 4A to 4C, FIGS.5A to 5F, FIGS. 6A to 6C, FIGS. 7A and 7B, FIGS. 8A to 8C, and FIGS. 9Ato 9C is described with reference to FIGS. 14A to 14C, FIG. 15, andFIGS. 16A to 16C. A deposition temperature in the deposition methodillustrated in FIGS. 14A to 14C, FIG. 15, and FIGS. 16A to 16C is higherthan that in the deposition method illustrated in FIGS. 4A to 4C, FIGS.5A to 5F, FIGS. 6A to 6C, FIGS. 7A and 7B, FIGS. 8A to 8C, and FIGS. 9Ato 9C. Zinc oxide is likely to vaporize at 600° C. or higher in areduced-pressure atmosphere. Therefore, oxide films formed using thedeposition method illustrated in FIGS. 4A to 4C, FIGS. 5A to 5F, FIGS.6A to 6C, FIGS. 7A and 7B, FIGS. 8A to 8C, and FIGS. 9A to 9C and thedeposition method illustrated in FIGS. 14A to 14C, FIG. 15, and FIGS.16A to 16C have different crystal structures.

As illustrated in FIG. 14A, the ions 15 collide with the sputteringtarget 13 and sputtered particles are released from the sputteringtarget 13 in a manner similar to that in FIG. 4A. Therefore, ions,sputtered particles, electrons, and/or the like are included in theplasma 17.

Examples of the sputtered particles include zinc particles, oxygenparticles, zinc oxide particles, In—Ga—Zn oxide particles, and the like.The sputtering target manufactured in Embodiment 1 contains Zn at ahigher ratio than that of Ga. For this reason, a description is givenhere using the model in which zinc particles, oxygen particles, or zincoxide particles are preferentially separated from the sputtering target13 and then zinc particles, oxygen particles, zinc oxide particles, andIn—Ga—Zn oxide particles are separated.

First, zinc particles 143 a and oxygen particles 143 b are separated assputtered particles from the sputtering target 13. Next, the zincparticles 143 a and the oxygen particles 143 b move to a substrate 141,whereby a hexagonal crystal grain 143 c of zinc oxide is formed over thesubstrate.

Note that because the substrate temperature here is 600° C. or higher,crystal growth occurs in the horizontal direction on the a-b planeparallel to a surface of the substrate 141. As a result, ahexagonal-crystal zinc oxide film 145 is formed as illustrated in FIG.14B. That is, the hexagonal-crystal zinc oxide film 145 includes asingle crystal region. Note that the hexagonal-crystal zinc oxide film145 is discontinuous because part of zinc oxide is vaporized unlike inthe deposition step illustrated in FIG. 4B.

FIG. 15 illustrates a model of a top-view shape of the hexagonal-crystalzinc oxide film 145. As illustrated in FIG. 15, Zn atoms and O atoms arebound in a hexagonal shape in the hexagonal-crystal zinc oxide film 145.The binding of Zn atoms and O atoms in the hexagonal shape extends overthe a-b plane.

Next, sputtered particles are released from the sputtering target. Here,In—Ga—Zn oxide particles are separated as the sputtered particles, andas illustrated in FIG. 14C, In—Ga—Zn oxide particles 147 are depositedover the hexagonal-crystal zinc oxide film 145, whereby a film 149including the In—Ga—Zn oxide particles is formed. The In—Ga—Zn oxideparticles 147 have a structure similar to that of the In—Ga—Zn oxideparticles 127. Note that zinc particles and oxygen particles, which arealso separated in this step as sputtered particles, are omitted here.

Here, the hexagonal-crystal zinc oxide film 145 formed over thesubstrate 141 is discontinuous and does not cover the substrate 141entirely. Therefore, the In—Ga—Zn oxide particle 147 is deposited overthe hexagonal-crystal zinc oxide film 145 so as to be aligned with thecrystal orientation of the hexagonal-crystal zinc oxide film 145.Specifically, the In—Ga—Zn oxide particle 147 separated from thesputtering target 13 is moved or rotated in the vicinity of thehexagonal-crystal zinc oxide film 145 such that the c-axis of theIn—Ga—Zn oxide particle 147 is parallel to the c-axis of thehexagonal-crystal zinc oxide film 145, and after that, the In—Ga—Znoxide particle 147 is deposited over the hexagonal-crystal zinc oxidefilm 145.

On the other hand, the In—Ga—Zn oxide particles 147 have random crystalorientations in a region where the hexagonal-crystal zinc oxide film 145is not formed.

Next, as illustrated in FIG. 16A, zinc particles 143 a and oxygenparticles 143 b are separated from the sputtering target and aretransferred and attached to the In—Ga—Zn oxide particles 147, in amanner similar to that in FIG. 14A. As a result, a hexagonal-crystalzinc oxide film 151 is formed as illustrated in FIG. 16B.

After that, sputtered particles are released from the sputtering targetin a manner similar to that in FIG. 14B, and In—Ga—Zn oxide particles153 are deposited over the hexagonal-crystal zinc oxide film 151 asillustrated in FIG. 16C. In addition, another In—Ga—Zn oxide particle isdeposited over the In—Ga—Zn oxide particles 153.

A highly crystalline oxide film can be formed by repeating the step offorming the hexagonal-crystal zinc oxide film 151 which is illustratedin FIG. 16B and the step of depositing the In—Ga—Zn oxide particles 153which is illustrated in FIG. 16C.

The atomic ratio of Ga to In (Ga/In) and the atomic ratio of Zn to In(Zn/In) in an In—Ga—Zn oxide formed by a sputtering method using thesputtering target of Embodiment 1 are lower than those in the sputteringtarget. The atomic ratio of Zn to Ga (Zn/Ga) in the In—Ga—Zn oxide filmis higher than or equal to 0.5.

Note that an oxide film obtained through the deposition processillustrated in FIGS. 14A to 14C, FIG. 15, and FIGS. 16A to 16C hasrandom crystal orientations and therefore has a polycrystallinestructure. However, sputtered particles deposited in the process offorming the film each have a homologous structure. Thus, the oxide filmobtained through the deposition process illustrated in FIGS. 14A to 14C,FIG. 15, and FIGS. 16A to 16C includes homologous structure regions andhas high crystallinity.

Through the above process, the In—Ga—Zn oxide film having apolycrystalline structure can be formed. Note that with use of Al, Ti,Y, Zr, La, Cs, Nd, or Hf as appropriate instead of Ga in the In—Ga—Znoxide, an In-M-Zn oxide (M represents Al, Ti, Y, Zr, La, Cs, Nd, or Hf)having a polycrystalline structure can be deposited.

Here, the deposited In-M-Zn oxide having a polycrystalline structure isdescribed. Note that the In-M-Zn oxide having a polycrystallinestructure is hereinafter referred to as a polycrystalline oxidesemiconductor. The polycrystalline oxide semiconductor includes aplurality of crystal grains.

In an image of a polycrystalline oxide semiconductor film which isobtained with a TEM, crystal grains can be found. In most cases, thesize of the crystal grains in the polycrystalline oxide semiconductorfilm is greater than or equal to 2 nm and less than or equal to 300 nm,greater than or equal to 3 nm and less than or equal to 100 nm, orgreater than or equal to 5 nm and less than or equal to 50 nm in animage obtained with the TEM, for example. Moreover, in an image of thepolycrystalline oxide semiconductor film which is obtained with the TEM,a boundary between crystal grains can be found in some cases.

For example, the polycrystalline oxide semiconductor film may include aplurality of crystal grains, and the plurality of crystal grains may beoriented in different directions. A polycrystalline oxide semiconductorfilm is subjected to structural analysis with an XRD apparatus. Forexample, when a polycrystalline oxide semiconductor film including anInGaZnO₄ crystal is analyzed by an out-of-plane method, peaks of 2θappear at around 31°, 36°, and the like in some cases.

For example, the polycrystalline oxide semiconductor film has highcrystallinity and thus has high electron mobility in some cases.Accordingly, a transistor including the polycrystalline oxidesemiconductor film as a channel formation region has high field-effectmobility. Note that there are cases in which an impurity is segregatedat the grain boundary in the polycrystalline oxide semiconductor film.Moreover, the grain boundary of the polycrystalline oxide semiconductorfilm becomes a defect state. Since the grain boundary of thepolycrystalline oxide semiconductor film may serve as a carrier trap ora carrier generation source, the transistor including thepolycrystalline oxide semiconductor film as a channel formation regionhas larger variation in electrical characteristics and lower reliabilitythan a transistor including a CAAC-OS film as a channel formation regionin some cases.

<Room-Temperature Deposition (Deposition Temperature: Higher than orEqual to 20° C. and Lower than or Equal to 150° C.)>

Next, a deposition method different from that in FIGS. 4A to 4C, FIGS.5A to 5F, FIGS. 6A to 6C, FIGS. 7A and 7B, FIGS. 8A to 8C, and FIGS. 9Ato 9C is described with reference to FIGS. 17A to 17C and FIGS. 18A and18B. A deposition temperature in the deposition method illustrated inFIGS. 17A to 17C and FIGS. 18A and 18B is lower than that in thedeposition method illustrated in FIGS. 4A to 4C, FIGS. 5A to 5F, FIGS.6A to 6C, FIGS. 7A and 7B, FIGS. 8A to 8C, and FIGS. 9A to 9C.

As illustrated in FIG. 17A, the ions 15 collide with the sputteringtarget 13 and sputtered particles are released from the sputteringtarget 13 in a manner similar to that in FIG. 4A. Therefore, ions,sputtered particles, electrons, and/or the like are included in theplasma 17.

Examples of the sputtered particles include zinc particles, oxygenparticles, zinc oxide particles, In—Ga—Zn oxide particles, and the like.The sputtering target manufactured in Embodiment 1 contains Zn at ahigher ratio than that of Ga. For this reason, a description is givenhere using the model in which zinc particles, oxygen particles, or zincoxide particles are preferentially separated from the sputtering target13 and then zinc particles, oxygen particles, zinc oxide particles, andIn—Ga—Zn oxide particles are separated.

First, zinc particles 163 a and oxygen particles 163 b are separated assputtered particles from the sputtering target 13 and are transferred toa substrate 161, whereby a zinc oxide film 165 is formed over thesubstrate 161 as illustrated in FIG. 17B.

Note that because the substrate temperature here is higher than or equalto 20° C. and lower than or equal to 150° C., the zinc oxide film 165has low crystallinity.

Next, sputtered particles are released from the sputtering target. Here,In—Ga—Zn oxide particles are separated as the sputtered particles, andas illustrated in FIG. 17C, In—Ga—Zn oxide particles 167 are depositedover the zinc oxide film 165, whereby a film 169 including the In—Ga—Znoxide particles is formed. The In—Ga—Zn oxide particles 167 have astructure similar to that of the In—Ga—Zn oxide particles 127. Note thatzinc particles and oxygen particles, which are also separated in thisstep as sputtered particles, are omitted here.

Since the zinc oxide film 165 has low crystallinity here, the In—Ga—Znoxide particles 167 deposited over the zinc oxide film 165 have randomcrystal orientations.

Then, as illustrated in FIG. 18A, zinc particles 163 a and oxygenparticles 163 b are separated from the sputtering target and aretransferred and attached to the In—Ga—Zn oxide particles 167. As aresult, a zinc oxide film with low crystallinity is formed. Furthermore,other sputtered particles as well as the zinc particles 163 a and theoxygen particles 163 b are deposited at the same time over the In—Ga—Znoxide particles 167 or the zinc oxide film with low crystallinity.Accordingly, a zinc oxide film 169 a with low crystallinity andsputtered particles 169 b are mixed as illustrated in FIG. 18B.

An In—Ga—Zn oxide film having a microcrystalline structure can be formedby repeating the step of transferring and attaching the zinc particles163 a and the oxygen particles 163 b to the In—Ga—Zn oxide particles 167which is illustrated in FIG. 18A and the step of depositing the In—Ga—Znoxide particles 169 b which is illustrated in FIG. 18B.

The atomic ratio of Ga to In (Ga/In) and the atomic ratio of Zn to In(Zn/In) in an In—Ga—Zn oxide formed by a sputtering method using thesputtering target of Embodiment 1 are lower than those in the sputteringtarget. The atomic ratio of Zn to Ga (Zn/Ga) in the In—Ga—Zn oxide filmis higher than or equal to 0.5.

Note that an oxide film obtained through the deposition processillustrated in FIGS. 17A to 17C and FIGS. 18A and 18B has random crystalorientations. Furthermore, the oxide film is deposited at a lowertemperature than that for the oxide film obtained through the depositionprocess illustrated in FIGS. 14A to 14C, FIG. 15, and FIGS. 16A to 16C,and therefore has lower crystallinity. However, sputtered particlesdeposited in the process of forming the film each have a homologousstructure. Thus, the oxide film obtained through the deposition processillustrated in FIGS. 17A to 17C and FIGS. 18A and 18B has highercrystallinity than an oxide film having an amorphous structure.

Through the above process, the In—Ga—Zn oxide film having amicrocrystalline structure can be formed. Note that with use of Al, Ti,Y, Zr, La, Cs, Nd, or Hf as appropriate instead of Ga in the In—Ga—Znoxide, an In-M-Zn oxide (M represents Al, Ti, Y, Zr, La, Cs, Nd, or Hf)having a microcrystalline structure can be deposited.

Here, the formed oxide film having a microcrystalline structure isdescribed below. Note that an In-M-Zn oxide having a microcrystallinestructure is hereinafter referred to as a microcrystalline oxidesemiconductor.

In an image of the microcrystalline oxide semiconductor film which isobtained with the TEM, it may be difficult to clearly find crystal partsin some cases. In most cases, the size of a crystal part in themicrocrystalline oxide semiconductor film is greater than or equal to 1nm and less than or equal to 100 nm, or greater than or equal to 1 nmand less than or equal to 10 nm, for example. A microcrystal with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, or a sizegreater than or equal to 1 nm and less than or equal to 3 nm, isspecifically referred to as nanocrystal (nc). An oxide semiconductorfilm including nanocrystal is referred to as an nc-OS (nanocrystallineoxide semiconductor) film. In an image of the nc-OS film which isobtained with the TEM, for example, it may be difficult to clearly finda crystal grain in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Furthermore, thereis no regularity of crystal orientation between different crystal partsin the nc-OS film. Thus, the orientation of the whole film is notobserved. Accordingly, in some cases, the nc-OS film cannot bedistinguished from an amorphous oxide semiconductor film depending on ananalysis method. For example, when the nc-OS film is subjected tostructural analysis by an out-of-plane method with an XRD apparatususing an X-ray having a diameter larger than the size of a crystal part,a peak which shows a crystal plane does not appear. Further, adiffraction pattern like a halo pattern appears in a selected-areaelectron diffraction pattern of the nc-OS film which is obtained byusing an electron beam having a probe diameter (e.g., larger than orequal to 50 nmφ) larger than the size of a crystal part. Meanwhile,spots are observed in an electron diffraction pattern of the nc-OS filmobtained by using an electron beam having a probe diameter (e.g., largerthan or equal to 1 nm and smaller than or equal to 30 nm) close to orsmaller than the size of a crystal part. Also in a nanobeam electrondiffraction pattern of the nc-OS film, a plurality of spots are shown ina ring-like region in some cases.

FIG. 10B shows an example of a nanobeam electron diffraction pattern ofa sample including an nc-OS film. The measurement position is changed.Here, the sample is cut in the direction perpendicular to a surfacewhere the nc-OS film is formed and the thickness thereof is reduced tobe less than or equal to 10 nm. Further, an electron beam with a probediameter of 1 nm enters from the direction perpendicular to the cutsurface of the sample. FIG. 10B shows that, when a nanobeam electrondiffraction is performed on the sample including the nc-OS film, adiffraction pattern exhibiting a crystal plane is obtained, butorientation along a crystal plane in a particular direction is notobserved.

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. Note that there is no regularity of crystalorientation between different crystal parts in the nc-OS film.Therefore, the nc-OS film has a higher density of defect states than theCAAC-OS film.

Accordingly, the nc-OS film has higher carrier density than the CAAC-OSfilm in some cases. An oxide semiconductor film with a high carrierdensity tends to have a high electron mobility. Therefore, a transistorincluding the nc-OS film as a channel formation region has a highfield-effect mobility in some cases. The nc-OS film has a higher densityof defect states than the CAAC-OS film, and thus may have a lot ofcarrier traps. Consequently, the transistor including the nc-OS film asa channel formation region has larger variation in electricalcharacteristics and lower reliability than a transistor including theCAAC-OS film as a channel formation region. Note that the nc-OS film canbe easily formed as compared to the CAAC-OS film because the nc-OS filmcan be obtained even when the amount of impurity contained therein isrelatively large; thus, the nc-OS film is sometimes preferably useddepending on the application. Therefore, a semiconductor deviceincluding the transistor including the nc-OS film can be manufacturedwith high productivity.

Embodiment 3

In this embodiment, a deposition apparatus for depositing a highlycrystalline oxide film will be described with reference to FIGS. 19 and20.

First, a structure of a deposition apparatus that hardly allows theentry of impurities into a film during deposition will be described withreference to FIGS. 19 and 20.

FIG. 19 is a schematic top view of a single wafer multi-chamberdeposition apparatus 4000. The deposition apparatus 4000 includes anatmosphere-side substrate supply chamber 4001 including a cassette port4101 for storing substrates and an alignment port 4102 for performingalignment of substrates, an atmosphere-side substrate transfer chamber4002 through which a substrate is transferred from the atmosphere-sidesubstrate supply chamber 4001, a load lock chamber 4003 a where asubstrate is carried in and the pressure is switched from atmosphericpressure to reduced pressure or from reduced pressure to atmosphericpressure, an unload lock chamber 4003 b where a substrate is carried outand the pressure is switched from reduced pressure to atmosphericpressure or from atmospheric pressure to reduced pressure, a transferchamber 4004 where a substrate is transferred in a vacuum, a substrateheating chamber 4005 where a substrate is heated, and depositionchambers 4006 a, 4006 b, and 4006 c in each of which a sputtering targetis placed for deposition.

Note that a plurality of cassette ports 4101 may be provided asillustrated in FIG. 19 (in FIG. 19, three cassette ports 4101 areprovided).

The atmosphere-side substrate transfer chamber 4002 is connected to theload lock chamber 4003 a and the unload lock chamber 4003 b, the loadlock chamber 4003 a and the unload lock chamber 4003 b are connected tothe transfer chamber 4004, and the transfer chamber 4004 is connected tothe substrate heating chamber 4005 and the deposition chambers 4006 a,4006 b, and 4006 c.

Note that gate valves 4104 are provided in connecting portions betweenthe chambers so that each chamber excluding the atmosphere-sidesubstrate supply chamber 4001 and the atmosphere-side substrate transferchamber 4002 can be independently kept in a vacuum state. In each of theatmosphere-side substrate supply chamber 4002 and the transfer chamber4004, a substrate transfer robot 4103 is provided, which is capable oftransferring glass substrates.

In the deposition apparatus 4000, substrates can be transferred withoutbeing exposed to the air between treatments, and adsorption ofimpurities to substrates can be suppressed. Note that the number oftransfer chambers, the number of deposition chambers, the number of loadlock chambers, the number of unload lock chambers, and the number ofsubstrate heating chambers are not limited to the above, and the numbersthereof can be set as appropriate depending on the space forinstallation or the process conditions.

FIG. 20 is a cross-section view taken along dashed-dotted line B1-B2 inthe deposition apparatus 4000 illustrated in FIG. 19.

A heating mechanism which can be used in the substrate heating chamber4005 may be a heating mechanism which uses a resistance heater or thelike for heating. Alternatively, it may be a heating mechanism whichuses heat conduction or heat radiation from a medium such as a heatedgas for heating. For example, rapid thermal annealing (RTA), such as gasrapid thermal annealing (GRTA) or lamp rapid thermal annealing (LRTA),can be used. In LRTA, an object is heated by radiation of light (anelectromagnetic wave) emitted from a lamp, such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressuresodium lamp, or a high-pressure mercury lamp. In GRTA, heat treatment isperformed using a high-temperature gas. As the gas, an inert gas isused.

The transfer chamber 4004 includes the substrate transfer robot 4103.The substrate transfer robot 4103 includes a plurality of movableportions and an arm for holding a substrate and can transfer a substrateto each chamber. Further, the transfer chamber 4004 is connected to avacuum pump 4200 and a cryopump 4201 through valves. With such astructure, the transfer chamber 4004 is evacuated from the atmosphericpressure to a low or medium vacuum (about 0.1 Pa to several hundredpascals) by using the vacuum pump 4200 and then evacuated from themedium vacuum to a high or ultrahigh vacuum (0.1 Pa to 1×10⁻⁷ Pa) byswitching between the valves and using the cryopump 4201.

Alternatively, two or more cryopumps 4201 may connected in parallel tothe transfer chamber 4004. With such a structure, even when one of thecryopumps is in regeneration, evacuation can be performed using any ofthe other cryopumps. Note that the above regeneration refers totreatment for discharging molecules (or atoms) trapped in the cryopump.When molecules (or atoms) are trapped too much in a cryopump, theevacuation capability of the cryopump is lowered; therefore,regeneration is performed regularly.

FIG. 20 shows a cross section of the deposition chamber 4006 b, thetransfer chamber 4004, and the load lock chamber 4003 a. The load lockchamber 4003 a includes a substrate delivery stage 4111.

Here, the details of the deposition chamber will be described withreference to FIG. 20. As the deposition chamber, a sputtering chamber, aplasma treatment chamber, or the like can be used as appropriate. Here,a description is given using a sputtering chamber as the depositionchamber. Note that the deposition chamber corresponds to the depositionchamber described in Embodiment 2. In the deposition chamber describedin Embodiment 2, the sputtering target and the substrate are illustratedas being placed horizontally, whereas in the deposition chamberdescribed in this embodiment, a sputtering target and a substrate areillustrated as being placed vertically.

The deposition chamber 4006 b illustrated in FIG. 20 includes asputtering target 4106, a deposition shield 4107, and a substrate stage4108. Note that the substrate stage 4108 here is provided with asubstrate 4109. Although not illustrated, the substrate stage 4108 maybe provided with a substrate holding mechanism for holding the substrate4109, a back side heater for heating the substrate 4109 from the backside, or the like.

A direct-current (DC) power source is preferably used as a power sourcefor applying a voltage to a sputtering target. Alternatively, a radiofrequency (RF) power source or an alternating-current (AC) power sourcecan be used. However, in the case of using a sputtering method with anRF power source, uniform plasma discharge to a large area is difficult.Therefore, it is sometimes inappropriate to employ a sputtering methodusing an RF power source for deposition on a large-sized substrate. Inaddition, a DC power source is preferred to an AC power source from thefollowing viewpoint.

In a sputtering method using a DC power source, a DC voltage is appliedbetween a sputtering target and a substrate as illustrated in FIG. 21A1,for example. Accordingly, the difference in potential between thesputtering target and the substrate during the DC voltage application isconstant as shown in FIG. 21B1. Thus, the sputtering method using a DCpower source can maintain constant plasma discharge.

In contrast, in a sputtering method using an AC power source, a cathodeand an anode switch between adjacent sputtering targets on the periodbasis (period A and period B) as illustrated in FIG. 21A2, for example.In period A in FIG. 21B2, for example, a sputtering target 1 functionsas a cathode and a sputtering target 2 functions as an anode. Further,in period B in FIG. 21B2, for example, the sputtering target 1 functionsas an anode and the sputtering target 2 functions as a cathode. The sumof period A and period B is approximately 20 microseconds to 50microseconds, for example. Thus, in the sputtering method using an ACpower source, plasma is discharged during alternating periods A and B.

Note that the substrate stage 4108 is held substantially vertically to afloor during deposition and is held substantially parallel to the floorwhen the substrate is delivered. In FIG. 20, the position where thesubstrate stage 4108 is held when the substrate is delivered is denotedby a dashed line. With such a structure, the probability that dust or aparticle which might be mixed into a film during deposition is attachedto the substrate 4109 can be lowered as compared to the case where thesubstrate stage 4108 is held parallel to the floor. However, there is apossibility that the substrate 4109 falls when the substrate stage 4108is held vertically (90°) to the floor; therefore, the angle of thesubstrate stage 4108 to the floor is preferred to be greater than orequal to 80° and lower than 90°.

The deposition shield 4107 can prevent sputtered particles separatedfrom the sputtering target 4106 from being deposited on a region wheredeposition is not necessary. Moreover, the deposition shield 4107 ispreferably processed to prevent accumulated sputtered particles frombeing separated. For example, blasting treatment which increases surfaceroughness may be performed, or roughness may be formed on the surface ofthe deposition shield 4107.

The deposition chamber 4006 b is connected to a mass flow controller4300 via a gas heating mechanism 4302, and the gas heating mechanism4302 is connected to a refiner 4301 via the mass flow controller 4300.With the gas heating mechanism 4302, gases to be introduced into thedeposition chamber 4006 b can be heated to a temperature higher than orequal to 40° C. and lower than or equal to 400° C., or higher than orequal to 50° C. and lower than or equal to 200° C. Note that althoughthe gas heating mechanism 4302, the mass flow controller 4300, and therefiner 4301 can be provided for each of a plurality of kinds of gases,only one gas heating mechanism 4302, one mass flow controller 4300, andone refiner 4301 are provided for simplicity. As the gas introduced intothe deposition chamber 4006 b, a gas whose dew point is −80° C. orlower, −100° C. or lower, or −120° C. or lower can be used; for example,an oxygen gas, a nitrogen gas, and a rare gas (e.g., an argon gas) areused.

The deposition chamber 4006 b is connected to a turbo molecular pump4202 and a vacuum pump 4200 via valves.

In addition, the deposition chamber 4006 b is provided with a cryotrap4110.

The cryotrap 4110 is a mechanism which can adsorb a molecule (or anatom) having a relatively high melting point, such as water. The turbomolecular pump 4202 is capable of stably evacuating a large-sizedmolecule (or atom), needs low frequency of maintenance, and thus enableshigh productivity, whereas it has a low capability in evacuatinghydrogen and water. Hence, the cryotrap 4110 is connected to thedeposition chamber 4006 b so as to have a high capability in evacuatingwater or the like. The temperature of a refrigerator of the cryotrap4110 is 100 K or lower, or 80 K or lower. In the case where the cryotrap4110 includes a plurality of refrigerators, it is preferable to set thetemperatures of the refrigerators at different temperatures becauseefficient evacuation is possible. For example, the temperatures of afirst-stage refrigerator and a second-stage refrigerator may be set to100 K or lower and 20 K or lower, respectively.

Note that the evacuation method of the deposition chamber 4006 b is notlimited to the above, and a structure similar to that in the evacuationmethod described in the transfer chamber 4004 (the evacuation methodusing the cryopump and the vacuum pump) may be employed. Needless tosay, the evacuation method of the transfer chamber 4004 may have astructure similar to that of the deposition chamber 4006 b (theevacuation method using the turbo molecular pump and the vacuum pump).

Note that in each of the above transfer chamber 4004, the substrateheating chamber 4005, and the deposition chamber 4006 b, the backpressure (total pressure) and the partial pressure of each gas molecule(atom) are preferably set as follows. In particular, the back pressureand the partial pressure of each gas molecule (atom) in the depositionchamber 4006 b need to be noted because impurities might enter a film tobe formed.

In each of the above chambers, the back pressure (total pressure) isless than or equal to 1×10⁻⁴ Pa, less than or equal to 3×10⁻⁵ Pa, orless than or equal to 1×10⁻⁵ Pa. In each of the above chambers, thepartial pressure of a gas molecule (atom) having a mass-to-charge ratio(m/z) of 18 is less than or equal to 3×10⁻⁵ Pa, less than or equal to1×10⁻⁵ Pa, or less than or equal to 3×10⁻⁶ Pa. Moreover, in each of theabove chambers, the partial pressure of a gas molecule (atom) having amass-to-charge ratio (m/z) of 28 is less than or equal to 3×10⁻⁵ Pa,less than or equal to 1×10⁻⁵ Pa, or less than or equal to 3×10⁻⁶ Pa.Moreover, in each of the above chambers, the partial pressure of a gasmolecule (atom) having a mass-to-charge ratio (m/z) of 44 is less thanor equal to 3×10⁻⁵ Pa, less than or equal to 1×10⁻⁵ Pa, or less than orequal to 3×10⁻⁶ Pa.

Note that a total pressure and a partial pressure in a vacuum chambercan be measured using a mass analyzer. For example, Qulee CGM-051, aquadrupole mass analyzer (also referred to as Q-mass) manufactured byULVAC, Inc. can be used.

Moreover, the above transfer chamber 4004, the substrate heating chamber4005, and the deposition chamber 4006 b preferably have a small amountof external leakage or internal leakage.

For example, in each of the above transfer chamber 4004, the substrateheating chamber 4005, and the deposition chamber 4006 b, the leakagerate is less than or equal to 3×10⁻⁶ Pa·m³/s, or less than or equal to1×10⁻⁶ Pa·m³/s. The leakage rate of a gas molecule (atom) having amass-to-charge ratio (m/z) of 18 is less than or equal to 1×10⁻⁷Pa·m³/s, or less than or equal to 3×10⁻⁸ Pa·m³/s. The leakage rate of agas molecule (atom) having a mass-to-charge ratio (m/z) of 28 is lessthan or equal to 1×10⁻⁵ Pa·m³/s, or less than or equal to 1×10⁻⁶Pa·m³/s. The leakage rate of a gas molecule (atom) having amass-to-charge ratio (m/z) of 44 is less than or equal to 3×10⁻⁶Pa·m³/s, or less than or equal to 1×10⁻⁶ Pa·m³/s.

Note that a leakage rate can be derived from the total pressure andpartial pressure measured using the mass analyzer.

When an oxide film is formed with the use of the above depositionapparatus, the entry of impurities to the oxide film can be suppressed.

Embodiment 4

In this embodiment, a semiconductor device which is one embodiment ofthe present invention and a manufacturing method thereof are describedwith reference to drawings.

In a transistor including an oxide semiconductor film, oxygen vacanciesare given as an example of a defect which leads to poor electricalcharacteristics of the transistor. For example, the threshold voltage ofa transistor including an oxide semiconductor film which contains oxygenvacancies in the film easily shifts in the negative direction, and sucha transistor tends to have normally-on characteristics. This is becauseelectric charges are generated owing to oxygen vacancies in the oxidesemiconductor film and the resistance is thus reduced. The transistorhaving normally-on characteristics causes various problems in thatmalfunction is likely to be caused when in operation and that powerconsumption is increased when not in operation, for example. Further,there is a problem in that the amount of change in electricalcharacteristics, typically in threshold voltage, of the transistor isincreased by change over time or a stress test.

One of the factors in generating oxygen vacancies is damage caused in amanufacturing process of a transistor. For example, when an insulatingfilm, a conductive film, or the like is formed over an oxidesemiconductor film by a plasma CVD method or a sputtering method, theoxide semiconductor film might be damaged depending on formationconditions thereof.

Another factor in generating oxygen vacancies is release of oxygen fromthe oxide semiconductor film due to heat treatment. For example, thereis a case where heat treatment is performed to remove impurities such ashydrogen, water, or the like contained in the oxide semiconductor film.When the heat treatment is performed with the oxide semiconductor filmexposed, oxygen is released from the oxide semiconductor film, therebyforming an oxygen vacancy.

Further, not only oxygen vacancies but also impurities such as siliconor carbon which is a constituent element of the insulating film causepoor electrical characteristics of a transistor. Therefore, there is aproblem in that mixing of the impurities into an oxide semiconductorfilm reduces the resistance of the oxide semiconductor film and theamount of change in electrical characteristics, typically in thresholdvoltage, of the transistor is increased by change over time or a stresstest.

Thus, an object of this embodiment is to reduce oxygen vacancies in anoxide semiconductor film serving as a channel region and theconcentration of impurities in the oxide semiconductor film, in asemiconductor device including a transistor having the oxidesemiconductor film.

Meanwhile, there is a trend in a commercially available display devicetoward a larger screen, e.g., a 60-inch diagonal screen, and further,the development of a display device is aimed even at a screen size of adiagonal of 120 inches or more. Hence, a glass substrate for a displaydevice has grown in size, e.g., to the 8th generation or more. However,in the case of using a large-sized substrate, because heat treatment isperformed at high temperatures, e.g., at 450° C. or higher, anexpensive, large-sized heating apparatus is needed. Accordingly, themanufacturing cost is increased. Further, high-temperature heattreatment causes a warp or a shrink of the substrate, which leads to areduction in yield.

Thus, one object of this embodiment is to manufacture a semiconductordevice using heat treatment at a temperature which allows the use of alarge-sized substrate and using a small number of heat treatment steps.

FIGS. 22A to 22C are a top view and cross-sectional views of atransistor 250 of a semiconductor device. The transistor 250 shown inFIGS. 22A to 22C is a channel-etched transistor. FIG. 22A is a top viewof the transistor 250, FIG. 22B is a cross-sectional view taken alongdashed-dotted line A-B in FIG. 22A, and FIG. 22C is a cross-sectionalview taken along dashed-dotted line C-D in FIG. 22A. Note that in FIG.22A, a substrate 211, one or more of components of the transistor 250(e.g., a gate insulating film 217), an oxide insulating film 223, anoxide insulating film 224, a nitride insulating film 225, and the likeare not illustrated for clarity.

The transistor 250 shown in FIGS. 22B and 22C includes a gate electrode215 provided over the substrate 211. Moreover, the gate insulating film217 over the substrate 211 and the gate electrode 215, an oxidesemiconductor film 218 overlapping with the gate electrode 215 with thegate insulating film 217 provided therebetween, and a pair of electrodes221 and 222 being in contact with the oxide semiconductor film 218 areincluded. Furthermore, a protective film 226 including the oxideinsulating film 223, the oxide insulating film 224, and the nitrideinsulating film 225 is formed over the gate insulating film 217, theoxide semiconductor film 218, and the pair of electrodes 221 and 222.

The transistor 250 described in this embodiment includes the oxidesemiconductor film 218. Further, part of the oxide semiconductor film218 serves as a channel region. Furthermore, the oxide insulating film223 is formed in contact with the oxide semiconductor film 218, and theoxide insulating film 224 is formed in contact with the oxide insulatingfilm 223.

The oxide semiconductor film 218 is typically an In-M-Zn oxide film (Mrepresents Al, Ti, Ga, Y, Zr, La, Cs, Nd, or Hf).

It is preferable that the atomic ratio of metal elements of a sputteringtarget used for forming a film of the In-M-Zn oxide satisfy In≧M andZn≧M As the atomic ratio of metal elements of such a sputtering target,In:M:Zn=1:1:1 and In:M:Zn=3:1:2 are preferable.

In the case where the oxide semiconductor film 218 is an In-M-Zn oxidefilm, the proportions of In and M when summation of In and M is assumedto be 100 atomic % are preferably as follows: the atomic percentage ofIn is greater than or equal to 25 atomic % and the atomic percentage ofM is less than 75 atomic %, or the atomic percentage of In is greaterthan or equal to 34 atomic % and the atomic percentage of M is less than66 atomic %.

The energy gap of the oxide semiconductor film 218 is 2 eV or more, 2.5eV or more, or 3 eV or more. With the use of an oxide semiconductorhaving such a wide energy gap, the off-state current of the transistor250 can be reduced.

The thickness of the oxide semiconductor film 218 is greater than orequal to 3 nm and less than or equal to 200 nm, greater than or equal to3 nm and less than or equal to 100 nm, or greater than or equal to 3 nmand less than or equal to 50 nm.

The oxide semiconductor film 218 is preferably formed using thesputtering target described in Embodiment 1, and typically, a sputteringtarget with an atomic ratio of In:M:Zn=1:1:1.05 to 1:1:1.5 can be used.Note that the atomic ratio of M to In and the atomic ratio of Zn to Inin the oxide semiconductor film 218 formed using such a sputteringtarget are lower than those in the sputtering target.

An In—Ga—Zn oxide film formed using such a sputtering target has ahomologous structure.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 218. For example, an oxide semiconductor filmwhose carrier density is 1×10¹⁷/cm³ or lower, 1×10¹⁵/cm³ or lower,1×10¹³/cm³ or lower, or 1×10¹¹/cm³ or lower is used as the oxidesemiconductor film 218.

Note that, without limitation to that described above, a material withan appropriate composition may be used depending on requiredsemiconductor characteristics and electrical characteristics (e.g.,field-effect mobility and threshold voltage) of a transistor. Further,in order to obtain required semiconductor characteristics of atransistor, it is preferable that the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementto oxygen, the interatomic distance, the density, and the like of theoxide semiconductor film 218 be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 218,an oxide semiconductor film in which the impurity concentration is lowand the density of defect states is low, in which case the transistorcan have more excellent electrical characteristics. Further, a highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has an extremely low off-state current; even when anelement has a channel width of 1×10⁶ μm and a channel length (L) of 10μm, the off-state current can be less than or equal to the measurementlimit of a semiconductor parameter analyzer, i.e., less than or equal to1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and adrain electrode of from 1 V to 10 V. Thus, the transistor whose channelregion is formed in the oxide semiconductor film has a small variationin electrical characteristics and high reliability in some cases.

Hydrogen contained in the oxide semiconductor film reacts with oxygenbonded to a metal atom to form water, and in addition, an oxygen vacancyis formed in a lattice from which oxygen is released (or a portion fromwhich oxygen is released). Due to entry of hydrogen into the oxygenvacancy, an electron serving as a carrier is generated in some cases.Further, in some cases, bonding of part of hydrogen to oxygen bonded toa metal element causes generation of an electron serving as a carrier.Thus, a transistor including an oxide semiconductor which containshydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible in the oxide semiconductor film 218. Specifically, the hydrogenconcentration of the oxide semiconductor film 218, which is measured bysecondary ion mass spectrometry (SIMS), is lower than or equal to 5×10¹⁹atoms/cm³, lower than or equal to 1×10¹⁹ atoms/cm³, lower than or equalto 5×10¹⁸ atoms/cm³, lower than or equal to 1×10¹⁸ atoms/cm³, lower thanor equal to 5×10¹⁷ atoms/cm³, or lower than or equal to 1×10¹⁶atoms/cm³.

When silicon or carbon which is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 218, oxygen vacancies areincreased, and the oxide semiconductor film 218 becomes an n-type film.Thus, the concentration of silicon or carbon (the concentration ismeasured by SIMS) of the oxide semiconductor film 218 is lower than orequal to 2×10¹⁸ atoms/cm³, or lower than or equal to 2×10¹⁷ atoms/cm³.

Further, the concentration of alkali metal or alkaline earth metal ofthe oxide semiconductor film 218, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, or lower than or equal to 2×10¹⁶atoms/cm³. Alkali metal and alkaline earth metal might generate carrierswhen bonded to an oxide semiconductor, in which case the off-statecurrent of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal of the oxide semiconductor film 218.

Further, when containing nitrogen, the oxide semiconductor film 218easily has n-type conductivity by generation of electrons serving ascarriers and an increase of carrier density. Thus, a transistorincluding an oxide semiconductor which contains nitrogen is likely to benormally on. For this reason, nitrogen in the oxide semiconductor filmis preferably reduced as much as possible; the concentration of nitrogenwhich is measured by SIMS is preferably set to, for example, lower thanor equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 218 is formed according to the depositionmodel described in Embodiment 2. The oxide semiconductor film 218 mayhave a non-single-crystal structure. Non-single-crystal structuresinclude the c-axis aligned crystalline oxide semiconductor (CAAC-OS),the polycrystalline structure, and the microcrystalline structure whichare described in Embodiment 2, and an amorphous structure, for example.Among the non-single-crystal structures, the amorphous structure has thehighest density of defect states, whereas CAAC-OS has the lowest densityof defect states. Therefore, the oxide semiconductor film 218 ispreferably CAAC-OS.

Note that the oxide semiconductor film 218 may be a mixed film includingtwo or more of the following: a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure. The mixed film has a single-layer structureincluding, for example, two or more of a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases. Further, the mixed film has astacked-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.

Furthermore, in the transistor 250 described in this embodiment, theoxide insulating film 223 is formed in contact with the oxidesemiconductor film 218, and the oxide insulating film 224 in contactwith the oxide insulating film 223 is formed.

The oxide insulating film 223 is an oxide insulating film which ispermeable to oxygen. Note that the oxide insulating film 223 also servesas a film which relieves damage to the oxide semiconductor film 218 atthe time of forming the oxide insulating film 224 later.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, or greater than or equal to 5 nm and less than or equal to 50 nm canbe used as the oxide insulating film 223. Note that in thisspecification, a “silicon oxynitride film” refers to a film thatcontains oxygen at a higher proportion than nitrogen, and a “siliconnitride oxide film” refers to a film that contains nitrogen at a higherproportion than oxygen.

Further, it is preferable that the amount of defects in the oxideinsulating film 223 be small, typically the spin density correspondingto a signal which appears at g=2.001 due to a dangling bond of silicon,be lower than or equal to 3×10¹⁷ spins/cm³ by ESR measurement. This isbecause if the density of defects in the oxide insulating film 223 ishigh, oxygen is bonded to the defects and the amount of oxygen thatpermeates the oxide insulating film 223 is decreased.

Further, it is preferable that the amount of defects at the interfacebetween the oxide insulating film 223 and the oxide semiconductor film218 be small, typically the spin density corresponding to a signal whichappears at g=1.93 due to an oxygen vacancy in the oxide semiconductorfilm 218 be lower than or equal to 1×10¹⁷ spins/cm³, more preferablylower than or equal to the lower limit of detection by ESR measurement.

The oxide insulating film 224 is formed in contact with the oxideinsulating film 223. The oxide insulating film 224 is formed using anoxide insulating film which contains oxygen at a higher proportion thanthe stoichiometric composition. Part of oxygen is released by heatingfrom the oxide insulating film which contains oxygen at a higherproportion than the stoichiometric composition. The oxide insulatingfilm containing oxygen at a higher proportion than the stoichiometriccomposition is an oxide insulating film of which the amount of releasedoxygen converted into oxygen atoms is greater than or equal to 1.0×10¹⁸atoms/cm³, or greater than or equal to 3.0×10²⁰ atoms/cm³ in TDSanalysis.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, or greater than or equal to 50 nm and less than or equal to 400 nmcan be used as the oxide insulating film 224.

Further, it is preferable that the amount of defects in the oxideinsulating film 224 be small, typically the spin density correspondingto a signal which appears at g=2.001 due to a dangling bond of silicon,be lower than 1.5×10¹⁸ spins/cm³, more preferably lower than or equal to1×10¹⁸ spins/cm³ by ESR measurement. Note that the oxide insulating film224 is provided more apart from the oxide semiconductor film 218 thanthe oxide insulating film 223 is; thus, the oxide insulating film 224may have higher defect density than the oxide insulating film 223.

Other details of the transistor 250 are described below.

There is no particular limitation on a material and the like of thesubstrate 211 as long as the material has heat resistance high enough towithstand at least heat treatment performed later. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate may be used as the substrate 211. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, an SOIsubstrate, or the like may be used. Still alternatively, any of thesesubstrates provided with a semiconductor element may be used as thesubstrate 211. In the case where a glass substrate is used as thesubstrate 211, a glass substrate having any of the following sizes canbe used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation(2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, alarge-sized display device can be manufactured.

Alternatively, a flexible substrate may be used as the substrate 211,and the transistor 250 may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate 211 and the transistor 250. The separation layer can be usedwhen part or the whole of a semiconductor device formed over theseparation layer is completed and separated from the substrate 211 andtransferred to another substrate. In such a case, the transistor 250 canbe transferred to a substrate having low heat resistance or a flexiblesubstrate as well.

The gate electrode 215 can be formed using a metal element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy containing any of these metal elements as acomponent; an alloy containing any of these metal elements incombination; or the like. Further, one or more metal elements selectedfrom manganese and zirconium may be used. The gate electrode 215 mayhave a single-layer structure or a stacked structure of two or morelayers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film or a tungsten nitride film, a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order, and the like can be given. Alternatively, an alloy filmor a nitride film which contains aluminum and one or more elementsselected from titanium, tantalum, tungsten, molybdenum, chromium,neodymium, and scandium may be used.

The gate electrode 215 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to have a stacked-layer structure formedusing the above light-transmitting conductive material and the abovemetal element.

The gate insulating film 217 can be formed to have a single-layerstructure or a stacked-layer structure using, for example, any ofsilicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, aluminum oxide, hafnium oxide, gallium oxide, Ga—Zn-based metaloxide, and the like.

The gate insulating film 217 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The thickness of the gate insulating film 217 is greater than or equalto 5 nm and less than or equal to 400 nm, greater than or equal to 10 nmand less than or equal to 300 nm, or greater than or equal to 50 nm andless than or equal to 250 nm.

The pair of electrodes 221 and 222 is formed to have a single-layerstructure or a stacked-layer structure including, as a conductivematerial, any of metals such as aluminum, titanium, chromium, nickel,copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungstenor an alloy containing any of these metals as its main component. Forexample, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a tungsten film, a two-layer structure in which a copperfilm is formed over a copper-magnesium-aluminum alloy film, athree-layer structure in which a titanium film or a titanium nitridefilm, an aluminum film or a copper film, and a titanium film or atitanium nitride film are stacked in this order, a three-layer structurein which a molybdenum film or a molybdenum nitride film, an aluminumfilm or a copper film, and a molybdenum film or a molybdenum nitridefilm are stacked in this order, and the like can be given. Note that atransparent conductive material containing indium oxide, tin oxide, orzinc oxide may be used.

Further, it is possible to prevent outward diffusion of oxygen from theoxide semiconductor film 218 and entry of hydrogen, water, or the likeinto the oxide semiconductor film 218 from the outside by providing thenitride insulating film 225 having a blocking effect against oxygen,hydrogen, water, alkali metal, alkaline earth metal, and the like overthe oxide insulating film 224. The nitride insulating film is formedusing silicon nitride, silicon nitride oxide, aluminum nitride, aluminumnitride oxide, or the like. Note that instead of the nitride insulatingfilm having a blocking effect against oxygen, hydrogen, water, alkalimetal, alkaline earth metal, and the like, an oxide insulating filmhaving a blocking effect against oxygen, hydrogen, water, and the like,may be provided. As the oxide insulating film having a blocking effectagainst oxygen, hydrogen, water, and the like, aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, and hafnium oxynitride can be given.

Next, a method for manufacturing the transistor 250 illustrated in FIGS.22A to 22C is described with reference to FIGS. 23A to 23D.

As illustrated in FIG. 23A, the gate electrode 215 is formed over thesubstrate 211, and the gate insulating film 217 is formed over the gateelectrode 215.

Here, a glass substrate is used as the substrate 211.

A method for forming the gate electrode 215 is described below. First, aconductive film is formed by a sputtering method, a CVD method, anevaporation method, or the like. Then, a mask is formed over theconductive film by a photolithography process. Next, part of theconductive film is etched with the use of the mask to form the gateelectrode 215. After that, the mask is removed.

Note that the gate electrode 215 may be formed by an electrolyticplating method, a printing method, an inkjet method, or the like insteadof the above formation method.

Here, a 100-nm-thick tungsten film is formed by a sputtering method.Next, a mask is formed by a photolithography process, and the tungstenfilm is subjected to dry etching with the use of the mask to form thegate electrode 215.

The gate insulating film 217 is formed by a sputtering method, a CVDmethod, an evaporation method, or the like.

Moreover, in the case of forming a gallium oxide film as the gateinsulating film 217, a metal organic chemical vapor deposition (MOCVD)method can be employed.

Here, the gate insulating film 217 is formed by stacking a 400-nm-thicksilicon nitride film and a 50-nm-thick silicon oxynitride film by aplasma CVD method.

Next, as illustrated in FIG. 23B, the oxide semiconductor film 218 isformed over the gate insulating film 217.

A formation method of the oxide semiconductor film 218 is describedbelow. An oxide semiconductor film which is to be the oxidesemiconductor film 218 is formed over the gate insulating film 217.Then, after a mask is formed over the oxide semiconductor film by aphotolithography process, the oxide semiconductor film is partly etchedusing the mask. Thus, the oxide semiconductor film 218 which is over thegate insulating film 217 and subjected to element isolation so as topartly overlap with the gate electrode 215 is formed as illustrated inFIG. 23B. After that, the mask is removed.

The oxide semiconductor film which is to be the oxide semiconductor film218 can be formed by a sputtering method, a coating method, a pulsedlaser deposition method, a laser ablation method, or the like.

Here, the oxide semiconductor film is formed by a sputtering methodusing the deposition apparatus described in Embodiment 3 which includesa sputtering chamber as a deposition chamber. In the sputtering chamber,a sputtering target manufactured in accordance with Embodiment 1 isplaced.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or amixed gas of a rare gas and oxygen is used as appropriate. In the caseof using the mixed gas of a rare gas and oxygen, the proportion ofoxygen is preferably higher than that of a rare gas.

In order to obtain a highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film, it is necessary to highlypurify a sputtering gas as well as to evacuate the chamber to a highvacuum. As an oxygen gas or an argon gas used for a sputtering gas, agas which is highly purified to have a dew point of −40° C. or lower,−80° C. or lower, −100° C. or lower, or −120° C. or lower is used,whereby entry of moisture or the like into the oxide semiconductor filmcan be minimized.

Here, a 35-nm-thick In—Ga—Zn oxide film is formed as the oxidesemiconductor film by a sputtering method using a sputtering target withan atomic ratio of In:Ga:Zn=1:1:1.05. Next, a mask is formed over theoxide semiconductor film, and part of the oxide semiconductor film isselectively etched. Thus, the oxide semiconductor film 218 is formed.

Next, the pair of electrodes 221 and 222 is formed as illustrated inFIG. 23C.

A method for forming the pair of electrodes 221 and 222 is describedbelow. First, a conductive film is formed by a sputtering method, a CVDmethod, an evaporation method, or the like. Then, a mask is formed overthe conductive film by a photolithography process. Next, the conductivefilm is etched with the use of the mask to form the pair of electrodes221 and 222. After that, the mask is removed.

Here, a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a100-nm-thick titanium film are sequentially stacked by a sputteringmethod. Next, a mask is formed over the titanium film by aphotolithography process and the tungsten film, the aluminum film, andthe titanium film are dry-etched with use of the mask to form the pairof electrodes 221 and 222.

Next, as shown in FIG. 23D, the oxide insulating film 223 is formed overthe oxide semiconductor film 218 and the pair of electrodes 221 and 222.Next, the oxide insulating film 224 is formed over the oxide insulatingfilm 223.

As the oxide insulating film 223, a silicon oxide film or a siliconoxynitride film can be formed under the following conditions: thesubstrate placed in a treatment chamber of a plasma CVD apparatus thatis vacuum-evacuated is held at a temperature higher than or equal to280° C. and lower than or equal to 400° C., the pressure is greater thanor equal to 20 Pa and less than or equal to 250 Pa, or greater than orequal to 100 Pa and less than or equal to 250 Pa with introduction of asource gas into the treatment chamber, and a high-frequency power issupplied to an electrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 223. Typicalexamples of the deposition gas containing silicon include silane,disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen,ozone, dinitrogen monoxide, nitrogen dioxide, and the like can be givenas examples.

With the use of the above conditions, an oxide insulating film which ispermeable to oxygen can be formed as the oxide insulating film 223.Further, by providing the oxide insulating film 223, damage to the oxidesemiconductor film 218 can be reduced in a step of forming the oxideinsulating film 224 which is formed later. Consequently, the amount ofoxygen vacancies in the oxide semiconductor film can be reduced.

Under the above film formation conditions, the bonding strength ofsilicon and oxygen becomes strong in the above substrate temperaturerange. Thus, as the oxide insulating film 223, a dense and hard oxideinsulating film which is permeable to oxygen, typically, a silicon oxidefilm or a silicon oxynitride film of which etching using hydrofluoricacid of 0.5 wt % at 25° C. is performed at a rate of lower than or equalto 10 nm/min, or lower than or equal to 8 nm/min can be formed.

Here, as the oxide insulating film 223, a 50-nm-thick silicon oxynitridefilm is formed by a plasma CVD method in which silane with a flow rateof 30 sccm and dinitrogen monoxide with a flow rate of 4000 sccm areused as a source gas, the pressure in the treatment chamber is 200 Pa,the substrate temperature is 220° C., and a high-frequency power of 150W is supplied to parallel-plate electrodes with the use of a 27.12 MHzhigh-frequency power source. Under the above conditions, a siliconoxynitride film which is permeable to oxygen can be formed.

As the oxide insulating film 224, a silicon oxide film or a siliconoxynitride film is formed under the following conditions: the substrateplaced in a treatment chamber of the plasma CVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 180°C. and lower than or equal to 280° C., or higher than or equal to 200°C. and lower than or equal to 240° C., the pressure is greater than orequal to 100 Pa and less than or equal to 250 Pa, or greater than orequal to 100 Pa and less than or equal to 200 Pa with introduction of asource gas into the treatment chamber, and a high-frequency power ofgreater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm²,or greater than or equal to 0.25 W/cm² and less than or equal to 0.35W/cm² is supplied to an electrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferablyused as the source gas of the oxide insulating film 224. Typicalexamples of the deposition gas containing silicon include silane,disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen,ozone, dinitrogen monoxide, nitrogen dioxide, and the like can be givenas examples.

Here, as the oxide insulating film 224, a 400-nm-thick siliconoxynitride film is formed by a plasma CVD method in which silane with aflow rate of 200 sccm and dinitrogen monoxide with a flow rate of 4000sccm are used as the source gas, the pressure in the treatment chamberis 200 Pa, the substrate temperature is 220° C., and the high-frequencypower of 1500 W is supplied to the parallel-plate electrodes with theuse of a 27.12 MHz high-frequency power source. Note that a plasma CVDapparatus used here is a parallel-plate plasma CVD apparatus in whichthe electrode area is 6000 cm², and the power per unit area (powerdensity) into which the supplied power is converted is 0.25 W/cm².

Next, heat treatment is performed. The heat treatment is performedtypically at a temperature of higher than or equal to 250° C. and lowerthan the strain point of the substrate, higher than or equal to 300° C.and lower than or equal to 550° C., or higher than or equal to 350° C.and lower than or equal to 510° C.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature of higher than or equal to the strainpoint of the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air with a water content of 20 ppm or less, 1 ppmor less, or 10 ppb or less), or a rare gas (argon, helium, or the like).The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gaspreferably does not contain hydrogen, water, and the like.

Note that as a heat treatment apparatus used for the heat treatment, theheating mechanism provided in the substrate heating chamber 4005described in Embodiment 3 can be used as appropriate.

By the heat treatment, part of oxygen contained in the oxide insulatingfilm 224 can be moved to the oxide semiconductor film 218, so that theamount of oxygen vacancies contained in the oxide semiconductor film 218can be further reduced.

Further, in the case where water, hydrogen, or the like is contained inthe oxide insulating film 223 and the oxide insulating film 224, whenthe nitride insulating film 225 having a function of blocking water,hydrogen, and the like is formed later and heat treatment is performed,water, hydrogen, or the like contained in the oxide insulating film 223and the oxide insulating film 224 is moved to the oxide semiconductorfilm 218, so that defects are generated in the oxide semiconductor film218. However, by the heating, water, hydrogen, or the like contained inthe oxide insulating film 223 and the oxide insulating film 224 can bereleased; thus, variation in electrical characteristics of thetransistor 250 can be reduced, and change in threshold voltage can beinhibited.

Note that when the oxide insulating film 224 is formed over the oxideinsulating film 223 while being heated, oxygen can be moved to the oxidesemiconductor film 218; thus, the heat treatment is not necessarilyperformed.

Here, heat treatment is performed at 350° C. for one hour in anatmosphere of nitrogen and oxygen.

Next, the nitride insulating film 225 is formed by a sputtering method,a CVD method, or the like.

Note that in the case where the nitride insulating film 225 is formed bya plasma CVD method, the substrate placed in the treatment chamber ofthe plasma CVD apparatus that is vacuum-evacuated is preferably set tobe higher than or equal to 300° C. and lower than or equal to 400° C.,more preferably, higher than or equal to 320° C. and lower than or equalto 370° C., so that a dense nitride insulating film can be formed.

Here, in the treatment chamber of a plasma CVD apparatus, a 50-nm-thicksilicon nitride film is formed by a plasma CVD method in which silanewith a flow rate of 50 sccm, nitrogen with a flow rate of 5000 sccm, andammonia with a flow rate of 100 sccm are used as the source gas, thepressure in the treatment chamber is 100 Pa, the substrate temperatureis 350° C., and high-frequency power of 1000 W is supplied toparallel-plate electrodes with the use of a 27.12 MHz high-frequencypower source. Note that the plasma CVD apparatus is a parallel-plateplasma CVD apparatus in which the electrode area is 6000 cm², and thepower per unit area (power density) into which the supplied power isconverted is 1.7×10⁻¹ W/cm².

By the above-described steps, the protective film 226 including theoxide insulating film 223, the oxide insulating film 224, and thenitride insulating film 225 can be formed.

Next, heat treatment may be performed. The heat treatment is performedtypically at a temperature of higher than or equal to 300° C. and lowerthan or equal to 400° C., or higher than or equal to 320° C. and lowerthan or equal to 370° C.

Through the above-described process, the transistor 250 can bemanufactured.

From the above, as for a semiconductor device including an oxidesemiconductor film, a semiconductor device in which the amount ofdefects is reduced can be obtained. Further, as for a semiconductordevice including an oxide semiconductor film, a semiconductor devicewith improved electrical characteristics can be obtained.

Modification Example 1 Regarding Base Insulating Film

In the transistor 250 described in this embodiment, a base insulatingfilm can be provided between the substrate 211 and the gate electrode215 as necessary. As a material of the base insulating film, siliconoxide, silicon oxynitride, silicon nitride, silicon nitride oxide,gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide, aluminumoxynitride, and the like can be given as examples. Note that whensilicon nitride, gallium oxide, hafnium oxide, yttrium oxide, aluminumoxide, or the like is used as a material of the base insulating film, itis possible to suppress diffusion of impurities such as alkali metal,water, and hydrogen into the oxide semiconductor film 218 from thesubstrate 211.

The base insulating film can be formed by a sputtering method, a CVDmethod, or the like.

Modification Example 2 Regarding Gate Insulating Film

In the transistor 250 described in this embodiment, the gate insulatingfilm 217 can have a stacked-layer structure as necessary.

The gate insulating film 217 can have a stacked-layer structure in whicha nitride insulating film and an oxide insulating film are stacked inthat order from the gate electrode 215 side. When the nitride insulatingfilm is provided on the gate electrode 215 side, an impurity, typicallyhydrogen, nitrogen, alkali metal, alkaline earth metal, or the like, canbe prevented from moving from the gate electrode 215 to the oxidesemiconductor film 218.

Further, when the oxide insulating film is provided on the oxidesemiconductor film 218 side, density of defect states at the interfacebetween the gate insulating film 217 and the oxide semiconductor film218 can be reduced. Consequently, a transistor whose electricalcharacteristics are hardly degraded can be obtained. Note that it ispreferable to form, as the oxide insulating film, an oxide insulatingfilm containing oxygen at a higher proportion than the stoichiometriccomposition like the oxide insulating film 224. This is because densityof defect states at the interface between the gate insulating film 217and the oxide semiconductor film 218 can be further reduced.

The gate insulating film 217 can have a stacked-layer structure in whicha nitride insulating film with few defects, a nitride insulating filmwith a high blocking property against hydrogen, and an oxide insulatingfilm are stacked in that order from the gate electrode 215 side. Whenthe nitride insulating film with few defects is provided in the gateinsulating film 217, the withstand voltage of the gate insulating film217 can be improved. Further, when the nitride insulating film with ahigh blocking property against hydrogen is provided, hydrogen can beprevented from moving from the gate electrode 215 and the nitrideinsulating film with few defects to the oxide semiconductor film 218.

The gate insulating film 217 can have a stacked-layer structure in whicha nitride insulating film with a high blocking property against animpurity, the nitride insulating film with few defects, the nitrideinsulating film with a high blocking property against hydrogen, and theoxide insulating film are stacked in that order from the gate electrode215 side. When the nitride insulating film with a high blocking propertyagainst an impurity is provided in the gate insulating film 217, animpurity, typically hydrogen, nitrogen, alkali metal, alkaline earthmetal, or the like, can be prevented from moving from the gate electrode215 to the oxide semiconductor film 218.

Modification Example 3 Regarding Pair of Electrodes

As for the pair of electrodes 221 and 222 provided in the transistor 250described in this embodiment, it is preferable to use a conductivematerial which is easily bonded to oxygen, such as tungsten, titanium,aluminum, copper, molybdenum, chromium, or tantalum, or an alloythereof. Thus, oxygen contained in the oxide semiconductor film 218 andthe conductive material contained in the pair of electrodes 221 and 222are bonded to each other, so that an oxygen deficient region is formedin the oxide semiconductor film 218. Further, in some cases, part ofconstituent elements of the conductive material that forms the pair ofelectrodes 221 and 222 is mixed into the oxide semiconductor film 218.Consequently, as shown in FIG. 24, low-resistance regions 220 a and 220b are formed in the vicinity of regions of the oxide semiconductor film218 which are in contact with the pair of electrodes 221 and 222. Thelow-resistance regions 220 a and 220 b are formed between the gateinsulating film 217 and the pair of electrodes 221 and 222 so as to bein contact with the pair of electrodes 221 and 222. Since thelow-resistance regions 220 a and 220 b have high conductivity, contactresistance between the oxide semiconductor film 218 and the pair ofelectrodes 221 and 222 can be reduced, and thus, the on-state current ofthe transistor can be increased.

Modification Example 4 Regarding Oxide Semiconductor Film

In the method for manufacturing the transistor 250 described in thisembodiment, after the pair of electrodes 221 and 222 is formed, theoxide semiconductor film 218 may be exposed to plasma generated in anoxygen atmosphere, so that oxygen may be supplied to the oxidesemiconductor film 218. Atmospheres of oxygen, ozone, dinitrogenmonoxide, nitrogen dioxide, and the like can be given as examples ofoxidizing atmospheres. Further, in the plasma treatment, the oxidesemiconductor film 218 is preferably exposed to plasma generated with nobias applied to the substrate 211 side. Consequently, the oxidesemiconductor film 218 can be supplied with oxygen without beingdamaged; accordingly, the amount of oxygen vacancies in the oxidesemiconductor film 218 can be reduced. Moreover, impurities, e.g.,halogen such as fluorine or chlorine remaining on the surface of theoxide semiconductor film 218 due to the etching treatment can beremoved.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in other embodiments,modification examples thereof, and examples.

Embodiment 5

In this embodiment, a semiconductor device having a transistor in whichthe amount of defects in an oxide semiconductor film can be furtherreduced as compared to Embodiment 4 is described with reference todrawings. The transistor described in this embodiment is different fromthat in Embodiment 4 in that a multilayer film having an oxidesemiconductor film and oxide in contact with the oxide semiconductorfilm is included.

FIGS. 25A to 25C are a top view and cross-sectional views of atransistor 260 included in the semiconductor device. FIG. 25A is a topview of the transistor 260, FIG. 25B is a cross-sectional view takenalong dashed-dotted line A-B in FIG. 25A, and FIG. 25C is across-sectional view taken along dashed-dotted line C-D in FIG. 25A.Note that in FIG. 25A, the substrate 211, one or more of components ofthe transistor 260 (e.g., the gate insulating layer 217), the oxideinsulating film 223, the oxide insulating film 224, the nitrideinsulating film 225, and the like are not illustrated for clarity.

The transistor 260 shown in FIGS. 25A to 25C includes a multilayer film220 overlapping with the gate electrode 215 with the gate insulatingfilm 217 provided therebetween, and the pair of electrodes 221 and 222in contact with the multilayer film 220. Furthermore, the protectivefilm 226 including the oxide insulating film 223, the oxide insulatingfilm 224, and the nitride insulating film 225 is formed over the gateinsulating film 217, the multilayer film 220, and the pair of electrodes221 and 222.

In the transistor 260 described in this embodiment, the multilayer film220 includes the oxide semiconductor film 218 and the oxide film 219.That is, the multilayer film 220 has a two-layer structure. Further,part of the oxide semiconductor film 218 serves as a channel region.Furthermore, the oxide insulating film 223 is formed in contact with themultilayer film 220, and the oxide insulating film 224 is formed incontact with the oxide insulating film 223. That is, the oxide film 219is provided between the oxide semiconductor film 218 and the oxideinsulating film 223.

The oxide film 219 is an oxide film containing one or more elementswhich form the oxide semiconductor film 218. Since the oxide film 219contains one or more elements which form the oxide semiconductor film218, interface scattering is unlikely to occur at the interface betweenthe oxide semiconductor film 218 and the oxide film 219. Thus, thetransistor can have a high field-effect mobility because the movement ofcarriers is not hindered at the interface.

The oxide film 219 is typically In—Ga oxide, In—Zn oxide, or In-M-Znoxide (M represents Al, Ti, Ga, Y, Zr, La, Cs, Nd, or Hf). The energy atthe conduction band bottom of the oxide film 219 is closer to a vacuumlevel than that of the oxide semiconductor film 218 is, and typically,the difference between the energy at the conduction band bottom of theoxide film 219 and the energy at the conduction band bottom of the oxidesemiconductor film 218 is any one of 0.05 eV or more, 0.07 eV or more,0.1 eV or more, and 0.15 eV or more, and any one of 2 eV or less, 1 eVor less, 0.5 eV or less, and 0.4 eV or less. That is, the differencebetween the electron affinity of the oxide film 219 and the electronaffinity of the oxide semiconductor film 218 is any one of 0.05 eV ormore, 0.07 eV or more, 0.1 eV or more, and 0.15 eV or more, and any oneof 2 eV or less, 1 eV or less, 0.5 eV or less, and 0.4 eV or less.

The oxide film 219 preferably contains In because carrier mobility(electron mobility) can be increased.

When the oxide film 219 contains a larger amount of Al, Ti, Ga, Y, Zr,La, Cs, Nd, or Hf in an atomic ratio than the amount of In in an atomicratio, any of the following effects may be obtained:

(1) the energy gap of the oxide film 219 is widened;

(2) the electron affinity of the oxide film 219 decreases;

(3) an impurity from the outside is blocked;

(4) an insulating property increases as compared to the oxidesemiconductor film 218; and

(5) oxygen vacancies are less likely to be generated in the oxide film219 containing a larger amount of Al, Ti, Ga, Y, Zr, La, Cs, Nd, or Hfin an atomic ratio than the amount of In in an atomic ratio because Al,Ti, Ga, Y, Zr, La, Cs, Nd, or Hf is a metal element which is stronglybonded to oxygen.

In the case where the oxide film 219 is an In-M-Zn oxide film (Mrepresents Al, Ti, Ga, Y, Zr, La, Cs, Nd, or Hf), the atomic ratio ofmetal elements of a sputtering target used for forming the In-M-Zn oxidefilm preferably satisfies M>In and Zn>M. As the atomic ratio of metalelements of such a sputtering target, In:Ga:Zn=1:3:4, In:Ga:Zn=1:3:5,In:Ga:Zn=1:3:6, In:Ga:Zn=1:3:7, In:Ga:Zn=1:3:8, In:Ga:Zn=1:3:9,In:Ga:Zn=1:3:10, In:Ga:Zn=1:4:5, In:Ga:Zn=1:4:6, In:Ga:Zn=1:4:7,In:Ga:Zn=1:4:8, In:Ga:Zn=1:4:9, In:Ga:Zn=1:4:10, In:Ga:Zn=1:5:6,In:Ga:Zn=1:5:7, In:Ga:Zn=1:5:8, In:Ga:Zn=1:5:9, In:Ga:Zn=1:5:10,In:Ga:Zn=1:6:7, In:Ga:Zn=1:6:8, In:Ga:Zn=1:6:9, or In:Ga:Zn=1:6:10 ispreferable.

In the case where the oxide film 219 is an In-M-Zn oxide film, theproportions of In and M when summation of In and M is assumed to be 100atomic % are preferably as follows: the atomic percentage of In is lessthan 50 atomic % and the atomic percentage of M is greater than or equalto 50 atomic %, or the atomic percentage of In is less than 25 atomic %and the atomic percentage of M is greater than or equal to 75 atomic %.

Further, in the case where each of the oxide semiconductor film 218 andthe oxide film 219 is an In-M-Zn oxide film (M represents Al, Ti, Ga, Y,Zr, La, Cs, Nd, or Hf), the proportion of M (M represents Al, Ti, Ga, Y,Zr, La, Cs, Nd, or Hf) in the oxide film 219 is higher than that in theoxide semiconductor film 218. Typically, the proportion of M in theoxide film 219 is 1.5 or more times, twice or more, or three or moretimes as high as that in the oxide semiconductor film 218.

Furthermore, in the case where each of the oxide semiconductor film 218and the oxide film 219 is an In-M-Zn oxide film (M represents Al, Ti,Ga, Y, Zr, La, Cs, Nd, or Hf), when In:M:Zn=x₁:y₁:z₁ [atomic ratio] issatisfied in the oxide film 219 and In:M:Zn=x₂:y₂:z₂ [atomic ratio] issatisfied in the oxide semiconductor film 218, y₁/x₁ is higher thany₂/x₂, or y₁/x₁ be 1.5 or more times as high as y₂/x₂. Alternatively,y₁/x₁ is twice or more as high as y₂/x₂, or y₁/x₁ is three or more timesas high as y₂/x₂. In this case, it is preferable that in the oxidesemiconductor film, y₂ be higher than or equal to x₂ because atransistor including the oxide semiconductor film can have stableelectrical characteristics. However, when y₂ is larger than or equal tothree or more times x₂, the field-effect mobility of the transistorincluding the oxide semiconductor film is reduced. Accordingly, y₂ ispreferably smaller than three times x₂.

A formation process which is similar to that of the oxide semiconductorfilm in Embodiment 4 can be used for the oxide semiconductor film 218.

The oxide film 219 is preferably formed using the sputtering targetdescribed in Embodiment 1, and typically, a sputtering target with anatomic ratio of In:M:Zn=1:3:3.05 to 1:3:10 or a sputtering target withan atomic ratio of In:M:Zn=1:6:6.05 to 1:6:10 can be used. Note that theatomic ratio of M/In and the atomic ratio of Zn/In in the oxide film 219formed using such a sputtering target are lower than those in thesputtering target. The atomic ratio of Zn to M (Zn/M) in an In—Ga—Znoxide film is higher than or equal to 0.5.

By a sputtering method using such a sputtering target, a film of anIn—Ga—Zn oxide that has a homologous structure and is CAAC-OS can beformed.

The oxide film 219 also serves as a film which relieves damage to theoxide semiconductor film 218 at the time of forming the oxide insulatingfilm 224 later. Consequently, the amount of oxygen vacancies in theoxide semiconductor film 218 can be reduced. In addition, by forming theoxide film 219, mixing of a constituent element of an insulating film,e.g., the oxide insulating film, formed over the oxide semiconductorfilm 218 to the oxide semiconductor film 218 can be inhibited.

The thickness of the oxide film 219 is greater than or equal to 3 nm andless than or equal to 100 nm, or greater than or equal to 3 nm and lessthan or equal to 50 nm.

The oxide film 219 may have a non-single-crystal structure, for example,like the oxide semiconductor film 218. Non-single-crystal structuresinclude the CAAC-OS which is described in Embodiment 2, apolycrystalline structure, a microcrystalline structure, and anamorphous structure, for example. Among the non-single-crystalstructures, the amorphous structure has the highest density of defectstates, whereas CAAC-OS has the lowest density of defect states.Therefore, the oxide film 219 is preferably CAAC-OS.

Note that the oxide semiconductor film 218 and the oxide film 219 mayeach be a mixed film including two or more of the following: a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure. The mixed filmhas a single-layer structure including, for example, two or more of aregion having an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Further, the mixed film has a stacked-layer structure including, forexample, two or more of a region having an amorphous structure, a regionhaving a microcrystalline structure, a region having a polycrystallinestructure, a CAAC-OS region, and a region having a single-crystalstructure in some cases. Furthermore, a microcrystalline structure andCAAC-OS may be stacked as the oxide semiconductor film 218 and the oxidefilm 219, respectively. Alternatively, the oxide semiconductor film 218may have a stacked-layer structure of a microcrystalline structure andCAAC-OS, and the oxide film 219 may be CAAC-OS.

It is preferable that the oxide semiconductor film 218 and the oxidefilm 219 each be CAAC-OS, in which case the crystallinity at theinterface between the oxide semiconductor film 218 and the oxide film219 can be increased.

Note that a channel formation region refers to a region in themultilayer film 220 which overlaps with the gate electrode 215 and ispositioned between the pair of electrodes 221 and 222. Further, achannel region refers to a region in the channel formation regionthrough which current mainly flows. Here, a channel region is part ofthe oxide semiconductor film 218 which is positioned between the pair ofelectrodes 221 and 222. A channel length refers to a distance betweenthe pair of electrodes 221 and 222.

Here, the oxide film 219 is provided between the oxide semiconductorfilm 218 and the oxide insulating film 223. Hence, if trap states areformed between the oxide film 219 and the oxide insulating film 223owing to impurities and defects, electrons flowing in the oxidesemiconductor film 218 are less likely to be captured by the trap statesbecause there is a distance between the trap states and the oxidesemiconductor film 218. Accordingly, the amount of on-state current ofthe transistor can be increased, and the field-effect mobility can beincreased. When electrons are captured by the trap states, the electronsbecome negative fixed charges. As a result, a threshold voltage of thetransistor changes. However, by the distance between the oxidesemiconductor film 218 and the trap states, capture of the electrons bythe trap states can be reduced, and accordingly a change of thethreshold voltage can be reduced.

Further, impurities from the outside can be blocked by the oxide film219, and accordingly, the amount of impurities which move from theoutside to the oxide semiconductor film 218 can be reduced. Further, anoxygen vacancy is less likely to be formed in the oxide film 219.Consequently, the impurity concentration and the amount of oxygenvacancies in the oxide semiconductor film 218 can be reduced.

Note that the oxide semiconductor film 218 and the oxide film 219 arenot formed by simply stacking each film, but are formed to form acontinuous junction (here, in particular, a structure in which theenergy of the bottom of the conduction band is changed continuouslybetween the films). In other words, a stacked-layer structure in whichthere exists no impurity which forms a defect level such as a trapcenter or a recombination center at each interface is provided. If animpurity exists between the oxide semiconductor film 218 and the oxidefilm 219 which are stacked, a continuity of the energy band is damaged,and the carrier is captured or recombined at the interface and thendisappears.

In order to form such a continuous junction it is necessary to formfilms continuously without being exposed to air, with use of themulti-chamber deposition apparatus including a load lock chamber whichis described in Embodiment 3.

As in a transistor 265 shown in FIG. 25D, a multilayer film 234overlapping with the gate electrode 215 with the gate insulating film217 provided therebetween, and the pair of electrodes 221 and 222 incontact with the multilayer film 234 may be included.

The multilayer film 234 includes an oxide film 231, the oxidesemiconductor film 218, and the oxide film 219. That is, the multilayerfilm 234 has a three-layer structure. The oxide semiconductor film 218serves as a channel region.

Further, the gate insulating film 217 and the oxide film 231 are incontact with each other. That is, the oxide film 231 is provided betweenthe gate insulating film 217 and the oxide semiconductor film 218.

The multilayer film 234 and the oxide insulating film 223 are in contactwith each other. The oxide insulating film 223 and the oxide insulatingfilm 224 are in contact with each other. That is, the oxide film 219 isprovided between the oxide semiconductor film 218 and the oxideinsulating film 223.

The oxide film 231 can be formed using a material and a formation methodof the oxide film 219 described in Embodiment 4.

When the thickness of the oxide film 231 is smaller than that of theoxide semiconductor film 218, the amount of change in threshold voltageof the transistor can be reduced.

In the transistor described in this embodiment, the oxide film 231 isprovided between the gate insulating film 217 and the oxidesemiconductor film 218, and the oxide film 219 is provided between theoxide semiconductor film 218 and the oxide insulating film 223. Thus, itis possible to reduce the concentration of silicon or carbon in thevicinity of the interface between the oxide film 231 and the oxidesemiconductor film 218, the concentration of silicon or carbon in theoxide semiconductor film 218, or the concentration of silicon or carbonin the vicinity of the interface between the oxide film 219 and theoxide semiconductor film 218.

Since the transistor 265 having such a structure includes very fewdefects in the multilayer film 234 including the oxide semiconductorfilm 218, the electrical characteristics of the transistor can beimproved, and typically, the on-state current can be increased and thefield-effect mobility can be improved. Further, in a BT stress test anda BT photostress test which are examples of a stress test, the amount ofchange in threshold voltage is small, and thus, reliability is high.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in other embodiments,modification examples thereof, and examples.

Embodiment 6

In this embodiment, a transistor having a different structure from thetransistors in Embodiments 4 and 5 will be described with reference toFIG. 26. A transistor 280 described in this embodiment includes aplurality of gate electrodes facing each other with an oxidesemiconductor film provided therebetween.

The transistor 280 shown in FIG. 26 includes the gate electrode 215provided over the substrate 211. Moreover, the gate insulating film 217over the substrate 211 and the gate electrode 215, the oxidesemiconductor film 218 overlapping with the gate electrode 215 with thegate insulating film 217 provided therebetween, and the pair ofelectrodes 221 and 222 being in contact with the oxide semiconductorfilm 218 are included. Furthermore, the protective film 226 includingthe oxide insulating film 223, the oxide insulating film 224, and thenitride insulating film 225 is formed over the gate insulating film 217,the oxide semiconductor film 218, and the pair of electrodes 221 and222. Further, a gate electrode 281 overlapping with the oxidesemiconductor film 218 with the protective film 226 providedtherebetween is included.

The gate electrode 281 can be formed in a manner similar to that of thegate electrode 215 described in Embodiment 4.

The transistor 280 described in this embodiment has the gate electrode215 and the gate electrode 281 facing each other with the oxidesemiconductor film 218 provided therebetween. By applying differentpotentials to the gate electrode 215 and the gate electrode 281, thethreshold voltage of the transistor 280 can be controlled.

Further, when the oxide semiconductor film 218 in which the amount ofoxygen vacancies is reduced is included, the electrical characteristicsof the transistor can be improved. Further, the transistor in which theamount of change in threshold voltage is small and which is highlyreliable is obtained.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in other embodiments,modification examples thereof, and examples.

Embodiment 7

In this embodiment, a transistor having a different structure from thetransistors in Embodiments 4 to 6 will be described with reference toFIGS. 27A to 27C.

In this embodiment, a semiconductor device having a transistor in whichthe amount of defects in an oxide semiconductor film can be furtherreduced as compared to Embodiments 4 to 6 is described with reference todrawings. The transistor described in this embodiment is different fromthose in Embodiments 4 to 6 in that the back channel side of the oxidesemiconductor film 218 is covered with the protective film and is notexposed to plasma generated in the etching treatment for forming thepair of electrodes.

FIGS. 27A to 27C are a top view and cross-sectional views of atransistor 290 included in the semiconductor device. FIG. 27A is a topview of the transistor 290, FIG. 27B is a cross-sectional view takenalong dashed-dotted line A-B in FIG. 27A, and FIG. 27C is across-sectional view taken along dashed-dotted line C-D in FIG. 27A.Note that in FIG. 27A, the substrate 211, one or more of components ofthe transistor 290 (e.g., the gate insulating layer 217), the oxideinsulating film 223, the oxide insulating film 224, the nitrideinsulating film 225, and the like are not illustrated for clarity.

The transistor 290 shown in FIGS. 27A to 27C includes the gate electrode215 provided over the substrate 211. Moreover, the gate insulating film217 over the substrate 211 and the gate electrode 215, and the oxidesemiconductor film 218 overlapping with the gate electrode 215 with thegate insulating film 217 provided therebetween are provided. Further,the protective film 226 including the oxide insulating film 223, theoxide insulating film 224, and the nitride insulating film 225 isprovided over the gate insulating film 217 and the oxide semiconductorfilm 218, and a pair of electrodes 221 b and 222 b which is formed overthe protective film 226 and is connected to the oxide semiconductor film218 in the opening of the protective film 226 is provided.

Next, a method for manufacturing the transistor 290 is described.

In a manner similar to Embodiment 4, the gate electrode 215 is formedover the substrate 211, and the gate insulating film 217 is formed overthe substrate 211 and the gate electrode 215. Next, the oxidesemiconductor film 218 is formed over the gate insulating film 217.

Next, in a manner similar to Embodiment 4, after the oxide insulatingfilm 223 is formed over the gate insulating film 217 and the oxidesemiconductor film 218 while heating is performed at a temperaturehigher than or equal to 220° C. and lower than or equal to 400° C., theoxide insulating film 224 and the nitride insulating film 225 areformed. Note that after the oxide insulating film 224 is formed, heattreatment is performed to supply part of oxygen contained in the oxideinsulating film 224 to the oxide semiconductor film 218.

Next, parts of the oxide insulating film 223, the oxide insulating film224, and the nitride insulating film 225 are etched to form an openingwhich exposes part of the oxide semiconductor film 218. After that, thepair of electrodes 221 b and 222 b in contact with the oxidesemiconductor film 218 is formed in a manner similar to Embodiment 4.

In this embodiment, the oxide semiconductor film 218 is covered with theprotective film 226 at the time of etching the pair of electrodes 221 band 222 b; thus, the oxide semiconductor film 218, particularly a backchannel region of the oxide semiconductor film 218, is not damaged bythe etching for forming the pair of electrodes 221 b and 222 b. Further,the oxide insulating film 224 is formed using an oxide insulating filmwhich contains oxygen at a higher proportion than the stoichiometriccomposition. Therefore, part of oxygen contained in the oxide insulatingfilm 224 can be moved to the oxide semiconductor film 218, so that theamount of oxygen vacancies in the oxide semiconductor film 218 can bereduced.

By the above-described process, defects contained in the oxidesemiconductor film 218 can be reduced, and thus, the reliability of thetransistor 290 can be improved.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in other embodiments,modification examples thereof, and examples.

Embodiment 8

In this embodiment, a transistor having a different structure from thetransistors in Embodiments 4 to 7 will be described with reference toFIGS. 28A to 28C.

In this embodiment, a semiconductor device having a transistor in whichthe amount of defects in an oxide semiconductor film can be furtherreduced as compared to Embodiments 4 to 7 is described with reference todrawings. The transistor described in this embodiment is different fromthose in Embodiments 4 to 7 in that the back channel side of the oxidesemiconductor film 218 is covered with the protective film and is notexposed to plasma generated in the etching treatment for forming thepair of electrodes in a manner similar to Embodiment 5.

FIGS. 28A to 28C are a top view and cross-sectional views of atransistor 295 included in the semiconductor device. The transistor 295shown in FIGS. 28A to 28C is a channel protective type transistor. FIG.28A is a top view of the transistor 295, FIG. 28B is a cross-sectionalview taken along dashed-dotted line A-B in FIG. 28A, and FIG. 28C is across-sectional view taken along dashed-dotted line C-D in FIG. 28A.Note that in FIG. 28A, the substrate 211 and one or more of componentsof the transistor 295 (e.g., the gate insulating film 217) are notillustrated for clarity.

The transistor 295 shown in FIGS. 28A to 28C includes the gate electrode215 over the substrate 211. Moreover, the gate insulating film 217 overthe substrate 211 and the gate electrode 215, and the oxidesemiconductor film 218 overlapping with the gate electrode 215 with thegate insulating film 217 provided therebetween are provided. Further, aprotective film 226 a including an oxide insulating film 223 a, an oxideinsulating film 224 a, and a nitride insulating film 225 a is providedover the gate insulating film 217 and the oxide semiconductor film 218,and a pair of electrodes 221 c and 222 c which is formed over the gateinsulating film 217, the oxide semiconductor film 218, and theprotective film 226 a is provided.

Next, a method for manufacturing the transistor 295 is described.

In a manner similar to Embodiment 4, the gate electrode 215 is formedover the substrate 211, and the gate insulating film 217 is formed overthe substrate 211 and the gate electrode 215. Next, the oxidesemiconductor film 218 is formed over the gate insulating film 217.

Next, in a manner similar to Embodiment 4, after the oxide insulatingfilm 223 is formed over the gate insulating film 217 and the oxidesemiconductor film 218 while heating is performed at a temperaturehigher than or equal to 220° C. and lower than or equal to 400° C., theoxide insulating film 224 and the nitride insulating film 225 areformed. Note that after the oxide insulating film 224 is formed, heattreatment is performed to supply part of oxygen contained in the oxideinsulating film 224 to the oxide semiconductor film 218.

Next, parts of the oxide insulating film 223, the oxide insulating film224, and the nitride insulating film 225 are etched to form theprotective film 226 a including the oxide insulating film 223 a, theoxide insulating film 224 a, and the nitride insulating film 225 a.

Next, the pair of electrodes 221 c and 222 c in contact with the oxidesemiconductor film 218 is formed in a manner similar to Embodiment 4.

In this embodiment, the oxide semiconductor film 218 is covered with theprotective film 226 a at the time of etching the pair of electrodes 221c and 222 c; thus, the oxide semiconductor film 218 is not damaged bythe etching for forming the pair of electrodes 221 c and 222 c. Further,the oxide insulating film 224 a is formed using an oxide insulating filmwhich contains oxygen at a higher proportion than the stoichiometriccomposition. Therefore, part of oxygen contained in the oxide insulatingfilm 224 a can be moved to the oxide semiconductor film 218, so that theamount of oxygen vacancies in the oxide semiconductor film 218 can bereduced.

Note that the nitride insulating film 225 a is formed in the protectivefilm 226 a in FIGS. 28A to 28C; however, the protective film 226 a mayhave a stacked-layer structure of the oxide insulating film 223 a andthe oxide insulating film 224 a. In that case, the nitride insulatingfilm 225 a is preferably formed after the pair of electrodes 221 c and222 c is formed. Thus, hydrogen, water, or the like can be preventedfrom entering the oxide semiconductor film 218 from the outside.

By the above-described process, defects contained in the oxidesemiconductor film 218 can be reduced, and thus, the reliability of thetransistor 295 can be improved.

Embodiment 9

In this embodiment, a top-gate transistor and a manufacturing methodthereof will be described.

FIGS. 29A to 29C are a top view and cross-sectional views of atransistor 400 of a semiconductor device. FIG. 29A is a top view of thetransistor 400, FIG. 29B is a cross-sectional view taken alongdashed-dotted line A-B in FIG. 29A, and FIG. 29C is a cross-sectionalview taken along dashed-dotted line C-D in FIG. 29A. Note that in FIG.29A, a substrate 401, an oxide insulating film 413, one or more ofcomponents of the transistor 400 (e.g., an oxide film 423, a gateinsulating film 425), an insulating film 427, an insulating film 429,and the like are not illustrated for clarity.

The transistor 400 illustrated in FIGS. 29A to 29C includes an oxidefilm 409 over the oxide insulating film 413 over the substrate 401, anoxide semiconductor film 411 over the oxide film 409, a pair ofelectrodes 415 and 416 in contact with the oxide semiconductor film 411,the oxide film 423 in contact with the oxide insulating film 413, theoxide semiconductor film 411, and the pair of electrodes 415 and 416,the gate insulating film 425 in contact with the oxide film 423, and agate electrode 421 overlapping with the oxide semiconductor film 411with the gate insulating film 425 provided therebetween. Note that theoxide film 409, the oxide semiconductor film 411, and the oxide film 423are collectively referred to as a multilayer film 424. The insulatingfilm 427 covering the gate insulating film 425 and the gate electrode421 and the insulating film 429 covering the insulating film 427 may beprovided. In openings 433 and 434 in the gate insulating film 425, theinsulating film 427, and the insulating film 429, wirings 431 and 432 incontact with the pair of electrodes 415 and 416 may be provided.

Components of the transistor 400 are described below.

As the substrate 401, the substrate 211 described in Embodiment 4 can beused as appropriate.

As a material of the oxide insulating film 413 serving as a baseinsulating film, silicon oxide, silicon oxynitride, silicon nitride,silicon nitride oxide, gallium oxide, hafnium oxide, yttrium oxide,aluminum oxide, aluminum oxynitride, and the like can be given asexamples. Note that when silicon nitride, gallium oxide, hafnium oxide,yttrium oxide, aluminum oxide, or the like is used as a material of theoxide insulating film 413 serving as a base insulating film, it ispossible to suppress diffusion of impurities such as alkali metal,water, and hydrogen into the oxide semiconductor film from the substrate401.

The oxide insulating film 413 can be formed using an oxide insulatingfilm which contains oxygen at a higher proportion than thestoichiometric composition. In other words, an oxide insulating filmfrom which part of oxygen is released by heating can be formed. With useof such a film, the oxygen in the oxide insulating film 413 istransferred to the oxide semiconductor film 411; thus, the density ofdefect states at the interface between the oxide insulating film 413 andthe oxide film 409 can be reduced, and oxygen vacancies can be furtherreduced by filling oxygen vacancies in the oxide semiconductor film 411.

For the oxide film 409 and the oxide film 423 included in the multilayerfilm 424, the material of the oxide films 219 and 231 in Embodiment 5can be used as appropriate, and for the oxide semiconductor film 411,the material of the oxide semiconductor film 218 in Embodiment 5 can beused as appropriate.

The thickness of the oxide semiconductor film 411 is greater than orequal to 3 nm and less than or equal to 200 nm, greater than or equal to3 nm and less than or equal to 100 nm, or greater than or equal to 3 nmand less than or equal to 50 nm.

The thickness of the oxide film 409 and the oxide film 423 is greaterthan or equal to 0.3 nm and less than or equal to 200 nm, greater thanor equal to 3 nm and less than or equal to 100 nm, or greater than orequal to 3 nm and less than or equal to 50 nm. Note that the thicknessof the oxide film 409 is preferably larger than that of the oxidesemiconductor film 411. The thickness of the oxide film 423 ispreferably smaller than that of the oxide semiconductor film 411.

In the case where the thickness of the oxide film 409 is too small,electrons are captured at the interface between the oxide film 409 andthe oxide semiconductor film 411, and the on-state current of thetransistor is decreased. On the other hand, in the case where thethickness of the oxide film 409 is too large, the amount of oxygentransferred from the oxide insulating film 413 to the oxidesemiconductor film 411 is decreased, and thus it is difficult to reducethe amount of oxygen vacancies in the oxide semiconductor film 411.Therefore, the thickness of the oxide film 409 is preferably larger thanthat of the oxide semiconductor film 411, and greater than or equal to20 nm and less than or equal to 200 nm. Furthermore, it is preferablethat the thickness of the oxide film 423 be smaller than that of theoxide semiconductor film 411 and the thickness of the oxide film 409 belarger than that of the oxide semiconductor film 411, in which case atransistor in which the amount of change in threshold voltage is smallcan be manufactured.

In the case where the oxide film 423 is a semiconductor, many electronsare induced in the oxide film 423. The oxide film 423 in which electronsare induced blocks an electric field of the gate electrode 421; thus, anelectric field applied to the oxide semiconductor film 411 is weakened.As a result, the on-state current of the transistor is decreased.Therefore, the thickness of the oxide film 423 is preferably smallerthan that of the oxide semiconductor film 411, and greater than or equalto 0.3 nm and less than or equal to 10 nm.

The oxide films 409 and 423 and the oxide semiconductor film 411 areformed according to the deposition model described in Embodiment 2. Theoxide films 409 and 423 and the oxide semiconductor film 411 may have anon-single-crystal structure. Non-single-crystal structures include theCAAC-OS, the polycrystalline structure, and the microcrystallinestructure which are described in Embodiment 2, and an amorphousstructure, for example. Among the non-single-crystal structures, theamorphous structure has the highest density of defect states, whereasCAAC-OS has the lowest density of defect states. Therefore, the oxidefilms 409 and 423 and the oxide semiconductor film 411 are eachpreferably CAAC-OS.

Note that the oxide films 409 and 423 and the oxide semiconductor film411 may each be a mixed film including two or more of the following: aregion having an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure. The mixed filmhas a single-layer structure including, for example, two or more of aregion having an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.Further, the mixed film has a stacked-layer structure including, forexample, two or more of a region having an amorphous structure, a regionhaving a microcrystalline structure, a region having a polycrystallinestructure, a CAAC-OS region, and a region having a single-crystalstructure in some cases. Furthermore, a microcrystalline structure,CAAC-OS, and CAAC-OS may be stacked as the oxide film 409, the oxidesemiconductor film 411, and the oxide film 423, respectively.Alternatively, the oxide film 409 may have a stacked-layer structure ofa microcrystalline structure and CAAC-OS, and the oxide semiconductorfilm 411 and the oxide film 423 may be CAAC-OS.

As the pair of electrodes 415 and 416, the pair of electrodes 221 and222 described in Embodiment 4 can be used as appropriate.

As the gate insulating film 425, the gate insulating film 217 describedin Embodiment 4 can be used as appropriate.

As the gate electrode 421, the gate electrode 215 described inEmbodiment 4 can be used as appropriate.

As the insulating films 427 and 429, the oxide insulating film 223 andthe oxide insulating film 224 described in Embodiment 4 can be used asappropriate. Note that an aluminum oxide film, a hafnium oxide film, ayttrium oxide film, or the like which can be used as an oxygen blockingfilm can be used as the insulating film 427.

In the case where side surfaces of the oxide film 423, the gateinsulating film 425, and the gate electrode 421 are substantiallyaligned with each other and the insulating film 427 is in contact withsurfaces of the pair of electrodes 415 and 416, the oxide film 423, thegate insulating film 425, and the gate electrode 421, release of oxygenfrom the multilayer film 424 in later heat treatment can be reduced.Thus, variation in electrical characteristics of the transistor can bereduced, and change in threshold voltage can be inhibited.

Although a stacked-layer structure of the insulating films 427 and 429is used here, a single-layer structure may be used.

The wirings 431 and 432 can be formed using a material similar to thatof the pair of electrodes 415 and 416, as appropriate.

In the transistor in this embodiment, an edge portion of the oxide film423 and an edge portion of the gate insulating film 425 aresubstantially aligned with an edge portion of the gate electrode 421.The oxide film 423 and the gate insulating film 425 having such shapescan be formed by forming the gate electrode 421 in FIG. 31A and etchingthe oxide film 417 and the gate insulating film 419 without an increasein the number of photomasks.

In the transistor 400, an etching residue generated at the time offorming the gate electrode 421 can be removed when the oxide film 423and the gate insulating film 425 are formed; thus, leakage currentgenerated between the gate electrode 421 and the wirings 431 and 432 canbe reduced.

A method for manufacturing the above semiconductor device is describedwith reference to FIGS. 30A to 30D and FIGS. 31A to 31C.

As illustrated in FIG. 30A, over the substrate 401, an oxide insulatingfilm 403 serving as a base insulating film is formed, and an oxide film405 and an oxide semiconductor film 407 are formed over the oxideinsulating film 403.

Here, a glass substrate is used as the substrate 401.

The oxide insulating film 403 can be formed by a sputtering method or aCVD method.

In the case where an oxide insulating film containing oxygen in excessof the stoichiometric composition is formed as the oxide insulating film403 in a manner similar to that of the oxide insulating film 224described in Embodiment 4, the oxide insulating film can be formed by aCVD method, a sputtering method, or the like. Alternatively, after theoxide insulating film is formed by a CVD method, a sputtering method, orthe like, oxygen may be added to the oxide insulating film by an ionimplantation method, an ion doping method, plasma treatment, or thelike.

Here, a 300-nm-thick silicon oxide film formed by a sputtering method isused as the oxide insulating film 403.

The oxide film 405 and the oxide semiconductor film 407 can be formed bya sputtering method, a coating method, a pulsed laser deposition method,a laser ablation method, or the like.

Here, a 20-nm-thick In—Ga—Zn oxide film is formed as the oxide film 405by a sputtering method using a sputtering target with an atomic ratio ofIn:Ga:Zn=1:3:4.

Next, by performing heat treatment, oxygen is preferably transferredfrom the oxide insulating film 403 to the oxide film 405 and the oxidesemiconductor film 407. Furthermore, impurities included in the oxidefilm 405 and the oxide semiconductor film 407 are preferably removed.

The heat treatment is performed typically at a temperature of higherthan or equal to 250° C. and lower than the strain point of thesubstrate, higher than or equal to 300° C. and lower than or equal to550° C., or higher than or equal to 350° C. and lower than or equal to510° C.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature of higher than or equal to the strainpoint of the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, ultra-dry air (air with a water content of 20 ppm or less, 1 ppmor less, or 10 ppb or less), or a rare gas (argon, helium, or the like).The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gaspreferably does not contain hydrogen, water, and the like.

Here, after heat treatment is performed at 450° C. for 1 hour in anitrogen atmosphere, heat treatment is performed at 450° C. for 1 hourin an oxygen atmosphere.

By the heat treatment, part of oxygen in the oxide insulating film 403serving as a base insulating film and the oxide film 409 to which oxygenis added is transferred to the oxide semiconductor film 407, so that theamount of oxygen vacancies in the oxide semiconductor film 407 isreduced. Note that in the oxide film 409 to which oxygen is added, theoxygen content is reduced by the heat treatment.

Through the above steps, oxygen vacancies in the oxide semiconductorfilm can be reduced.

Note that the heat treatment may be performed in a later step, not inthis step. In other words, in another heating step performed later, partof oxygen in the oxide insulating film 403 may be transferred to theoxide semiconductor film 407. In this case, the number of heating stepscan be reduced.

Then, after a mask is formed over the oxide semiconductor film 407 by aphotolithography process, the oxide film 409 and the oxide semiconductorfilm 407 are each partly etched using the mask. Accordingly, the oxidefilm 409 and the oxide semiconductor film 411 are formed as illustratedin FIG. 30B. After that, the mask is removed. Note that in the etchingstep, the oxide insulating film 403 is partly etched in some cases.Here, the oxide insulating film 403 which is partly etched is referredto as the oxide insulating film 413.

Next, as illustrated in FIG. 30C, the pair of electrodes 415 and 416 isformed over the oxide semiconductor film 411.

A method for forming the pair of electrodes 415 and 416 is describedbelow. First, a conductive film is formed by a sputtering method, a CVDmethod, an evaporation method, or the like. Then, a mask is formed overthe conductive film by a photolithography process. Next, the conductivefilm is etched with the use of the mask to form the pair of electrodes415 and 416. After that, the mask is removed.

Here, a 50-nm-thick titanium film, a 400-nm-thick aluminum film, and a100-nm-thick titanium film are sequentially stacked by a sputteringmethod. Next, a mask is formed over the titanium film by aphotolithography process and the titanium film, the aluminum film, andthe titanium film are dry-etched with use of the mask to form the pairof electrodes 415 and 416.

After the pair of electrodes 415 and 416 is formed, cleaning treatmentis preferably performed to remove an etching residue. A short circuit ofthe pair of electrodes 415 and 416 can be suppressed by this cleaningtreatment. The cleaning treatment can be performed using an alkalinesolution such as a tetramethylammonium hydroxide (TMAH) solution, or anacidic solution such as diluted hydrofluoric acid, an oxalic acidsolution, or a phosphoric acid solution.

Next, as illustrated in FIG. 30D, an oxide film 417 is formed over theoxide semiconductor film 411 and the pair of electrodes 415 and 416, anda gate insulating film 419 is formed over the oxide film 417.

The oxide film 417 can be formed in a manner similar to that of theoxide film 409. The gate insulating film 419 can be formed by asputtering method, a CVD method, or the like.

Here, a 5-nm-thick In—Ga—Zn oxide film is formed as the oxide film 417by a sputtering method using a sputtering target with an atomic ratio ofIn:Ga:Zn=1:3:4.

Then, as illustrated in FIG. 31A, the gate electrode 421 is formed in aregion which is over the gate insulating film 419 and overlaps with theoxide semiconductor film 411.

A method for forming the gate electrode 421 is described below. First, aconductive film is formed by a sputtering method, a CVD method, anevaporation method, or the like. Then, a mask is formed over theconductive film by a photolithography process. Next, part of theconductive film is etched with the use of the mask to form the gateelectrode 421. After that, the mask is removed.

Note that the gate electrode 421 may be formed by an electrolyticplating method, a printing method, an inkjet method, or the like insteadof the above formation method.

Here, a 15-nm-thick tantalum nitride film and a 135-nm-thick tungstenfilm are formed in this order by a sputtering method. Next, a mask isformed by a photolithography process, and the tantalum nitride film andthe tungsten film are subjected to dry etching with the use of the maskto form the gate electrode 421.

Next, as illustrated in FIG. 31B, the oxide film 417 and the gateinsulating film 419 are etched using the gate electrode 421 as a mask toform the oxide film 423 and the gate insulating film 425. In thismanner, the oxide film 423 and the gate insulating film 425 can beformed without an increase in the number of photomasks. Furthermore, anedge portion of the oxide film 423 and an edge portion of the gateinsulating film 425 are substantially aligned with an edge portion ofthe gate electrode 421.

In the transistor 450, an etching residue generated at the time offorming the gate electrode 421 can be removed when the oxide film 423and the gate insulating film 425 are formed; thus, leakage currentgenerated between the gate electrode 421 and the wirings 431 and 432which are formed later can be reduced.

Next, as illustrated in FIG. 31C, the insulating film 427 and theinsulating film 429 are stacked in this order over the pair ofelectrodes 415 and 416 and the gate electrode 421. Next, heat treatmentis performed. After openings are formed in the insulating film 427 andthe insulating film 429, the wirings 431 and 432 are formed.

The insulating film 427 and the insulating film 429 can be formed by asputtering method, a CVD method, or the like as appropriate. When anoxygen blocking film is used as the insulating film 427, release ofoxygen from the multilayer film 424 in later heat treatment can bereduced. Thus, variation in electrical characteristics of the transistorcan be reduced, and change in threshold voltage can be inhibited.

Here, a 300-nm-thick silicon oxynitride film is formed by a plasma CVDmethod as the insulating film 427, and a 50-nm-thick silicon nitridefilm is formed by a sputtering method as the insulating film 429.

The heat treatment is performed typically at a temperature of higherthan or equal to 150° C. and lower than the strain point of thesubstrate, higher than or equal to 250° C. and lower than or equal to500° C., or higher than or equal to 300° C. and lower than or equal to450° C. Here, heat treatment is performed at 350° C. for one hour in anatmosphere of nitrogen and oxygen.

The wirings 431 and 432 can be formed in a manner similar to that of thepair of electrodes 415 and 416. Alternatively, the wirings 431 and 432can be formed by a damascene method.

Through the above steps, a transistor which has a multilayer filmincluding an oxide semiconductor film and having a low density oflocalized levels and which has excellent electrical characteristics canbe manufactured. In addition, a highly reliable transistor in which avariation in electrical characteristics with time or a variation inelectrical characteristics due to a stress test is small can bemanufactured.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in other embodiments,modification examples thereof, and examples.

Embodiment 10

Although the variety of films such as the metal films, the oxidesemiconductor films, and the inorganic insulating films which aredescribed in the above embodiments can be formed by a sputtering methodor a plasma chemical vapor deposition (CVD) method, such films may beformed by another method, e.g., a thermal CVD method. A metal organicchemical vapor deposition (MOCVD) method or an atomic layer deposition(ALD) method may be employed as an example of a thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a time,the pressure in a chamber is set to an atmospheric pressure or a reducedpressure, and reaction is caused in the vicinity of the substrate orover the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first source gas so thatthe source gases are not mixed, and then a second source gas isintroduced. Note that in the case where the first source gas and theinert gas are introduced at a time, the inert gas serves as a carriergas, and the inert gas may also be introduced at the same time as theintroduction of the second source gas. Alternatively, the first sourcegas may be exhausted by vacuum evacuation instead of the introduction ofthe inert gas, and then the second source gas may be introduced. Thefirst source gas is adsorbed on the surface of the substrate to form afirst layer; then the second source gas is introduced to react with thefirst layer; as a result, a second layer is stacked over the firstlayer, so that a thin film is formed. The sequence of the gasintroduction is repeated a plurality of times until a desired thicknessis obtained, whereby a thin film with excellent step coverage can beformed. The thickness of the thin film can be adjusted by the number ofrepetition times of the sequence of the gas introduction; therefore, anALD method makes it possible to accurately adjust a thickness and thusis suitable for manufacturing a minute FET.

The variety of films such as the metal film, the oxide semiconductorfilm, and the inorganic insulating film which are described in the aboveembodiments can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, in the case where an In—Ga—Zn—O film isformed, trimethylindium, trimethylgallium, and dimethylzinc are used.Note that the chemical formula of trimethylindium is In(CH₃)₃. Thechemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formulaof dimethylzinc is Zn(CH₃)₂. Without limitation to the abovecombination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be usedinstead of trimethylgallium and diethylzinc (chemical formula:Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed using adeposition apparatus employing ALD, two kinds of gases, i.e., ozone (O₃)as an oxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and a hafnium precursor compound (a hafniumalkoxide solution, typically tetrakis(dimethylamide)hafnium (TDMAH)) areused. Note that the chemical formula of tetrakis(dimethylamide)hafniumis Hf[N(CH₃)₂]₄. Examples of another material liquid includetetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed using adeposition apparatus employing ALD, two kinds of gases, e.g., H₂O as anoxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed using adeposition apparatus employing ALD, hexachlorodisilane is adsorbed on asurface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced a plurality of times to form an initial tungstenfilm, and then a WF₆ gas and an H₂ gas are introduced at a time, so thata tungsten film is formed. Note that an SiH₄ gas may be used instead ofa B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed using a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas are sequentially introduced a plurality oftimes to form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are introducedat a time to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed bymixing of these gases. Note that although an H₂O gas which is obtainedby bubbling with an inert gas such as Ar may be used instead of an O₃gas, it is preferable to use an O₃ gas, which does not contain H.Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of aGa(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Further, instead of anIn(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gasmay be used.

Note that the structures, methods, and the like described in thisembodiment can be used as appropriate in combination with any of thestructures, methods, and the like described in other embodiments,modification examples thereof, and examples.

Example 1

In this example, In—Ga—Zn oxide films were formed using sputteringtargets each containing an In—Ga—Zn oxide. The atomic ratios and thecrystal structures of the sputtering targets and the films formed willbe described.

<Structure of Sample>

First, a method for forming samples will be described.

In this example, each sample was formed by forming a 100-nm-thickIn—Ga—Zn oxide film over a quartz substrate.

Here, the In—Ga—Zn oxide film was formed using a sputtering targetcontaining an In—Ga—Zn oxide under the conditions where a mixed gas ofoxygen and argon at a flow rate ratio of 1:2 was supplied as asputtering gas to a deposition chamber, the pressure inside thedeposition chamber was controlled to 0.4 Pa, and a DC power of 200 W wassupplied.

Note that a sample including an In—Ga—Zn oxide film formed using asputtering target containing an In—Ga—Zn oxide including metal elementsat an atomic ratio of In:Ga:Zn=1:3:2 is referred to as a sample 1. Notethat the substrate temperature was set at 200° C.

Note that a sample including an In—Ga—Zn oxide film formed using asputtering target containing an In—Ga—Zn oxide including metal elementsat an atomic ratio of In:Ga:Zn=1:3:4 is referred to as a sample 2. Notethat the substrate temperature was set at 200° C.

Note that a sample including an In—Ga—Zn oxide film formed using asputtering target containing an In—Ga—Zn oxide including metal elementsat an atomic ratio of In:Ga:Zn=1:3:6 is referred to as a sample 3. Notethat the substrate temperature was set at 200° C.

Note that a sample including an In—Ga—Zn oxide film formed using asputtering target containing an In—Ga—Zn oxide including metal elementsat an atomic ratio of In:Ga:Zn=1:6:2 is referred to as a sample 4. Notethat the substrate temperature was set at 200° C.

Note that a sample including an In—Ga—Zn oxide film formed using asputtering target containing an In—Ga—Zn oxide including metal elementsat an atomic ratio of In:Ga:Zn=1:6:4 is referred to as a sample 5. Notethat the substrate temperature was set at 200° C.

Note that a sample including an In—Ga—Zn oxide film formed using asputtering target containing an In—Ga—Zn oxide including metal elementsat an atomic ratio of In:Ga:Zn=1:6:8 is referred to as a sample 6. Notethat the substrate temperature was set at 200° C.

Note that a sample including an In—Ga—Zn oxide film formed using asputtering target containing an In—Ga—Zn oxide including metal elementsat an atomic ratio of In:Ga:Zn=1:6:10 is referred to as a sample 7. Notethat the substrate temperature was set at 200° C.

Note that a sample including an In—Ga—Zn oxide film formed using asputtering target containing an In—Ga—Zn oxide including metal elementsat an atomic ratio of In:Ga:Zn=0:2:1 is referred to as a sample 8. Notethat the substrate temperature was set at 200° C.

Note that a sample including an In—Ga—Zn oxide film formed using asputtering target containing an In—Ga—Zn oxide including metal elementsat an atomic ratio of In:Ga:Zn=3:1:2 is referred to as a sample 9. Notethat the substrate temperature was set at 200° C.

Note that a sample including an In—Ga—Zn oxide film formed using asputtering target containing an In—Ga—Zn oxide including metal elementsat an atomic ratio of In:Ga:Zn=1:1:1 is referred to as a sample 10. Notethat the substrate temperature was set at 300° C.

<XPS>

Next, the atomic ratios of the metal elements included in the In—Ga—Znoxide films of the samples 1 to 10 and those in the sputtering targetsused to form the samples 1 to 10 were quantified by X-ray photoelectronspectroscopy (XPS) analysis. FIG. 32 shows the atomic ratios of Ga to Inand the atomic ratios of Zn to In. Note that in FIG. 32, the atomicratio of In:Ga:Zn in each sputtering target used for deposition is inparentheses.

FIG. 32 shows that the atomic ratio of Ga to In and the atomic ratio ofZn to In in each In—Ga—Zn oxide film formed are lower than the atomicratio of Ga to In and the atomic ratio of Zn to In in each sputteringtarget.

<XRD>

Next, the crystal structures of the In—Ga—Zn oxide films included in thesamples 1 to 10 were measured by X-ray diffraction (XRD). Note that thesamples 1 to 10 were each formed under two conditions where thesputtering gas for forming the In—Ga—Zn oxide film was a mixed gas ofoxygen and argon at a flow rate ratio of 1:2 and where the sputteringgas was only oxygen.

FIGS. 33A and 33B, FIGS. 34A and 34B, and FIGS. 35A and 35B show resultsof X-ray diffraction measurement of the In—Ga—Zn oxide films formed. Astypical examples, results of X-ray diffraction measurement of the sample1, the sample 2, and the sample 3 are shown in FIGS. 33A and 33B, FIGS.34A and 34B, and FIGS. 35A and 35B, respectively. Note that the In—Ga—Znoxide films of the samples 1 to 3 which were formed using only oxygen asa sputtering gas were measured.

FIGS. 33A, 34A, and 35A each show an XRD spectrum measured by anout-of-plane method, where the vertical axis represents X-raydiffraction intensity (arbitrary unit) and the horizontal axisrepresents diffraction angle 2θ (deg.). FIGS. 33B, 34B, and 35B eachshow an XRD spectrum measured by an in-plane method, where the verticalaxis represents X-ray diffraction intensity (arbitrary unit) and thehorizontal axis represents diffraction angle 2θ (deg.).

In FIGS. 33A and 33B, FIGS. 34A and 34B, and FIGS. 35A and 35B, theletter “h” represents a homologous structure, and the letter “s”represents a spinel structure. The orientation of a plane of a crystalstructure is in parentheses.

The In—Ga—Zn oxide film of the sample 1 shown in FIGS. 33A and 33B has aspinel structure. The In—Ga—Zn oxide film of the sample 2 shown in FIGS.34A and 34B has a spinel structure and a homologous structure. TheIn—Ga—Zn oxide film of the sample 3 shown in FIGS. 35A and 35B has ahomologous structure.

FIGS. 33A and 33B, FIGS. 34A and 34B, and FIGS. 35A and 35B suggest thatan In—Ga—Zn oxide film having a homologous structure can be formed byusing a sputtering target containing Zn at a higher ratio than that ofGa.

FIG. 36 shows a ternary phase diagram where the composition ratio of Into Ga and Zn in the In—Ga—Zn oxide film of each sample and that in thesputtering target used to form the sample are plotted on the basis ofthe results of XPS measurement in FIG. 32 and the results of X-raydiffraction measurement in FIGS. 33A and 33B, FIGS. 34A and 34B, andFIGS. 35A and 35B. Note that in FIG. 36, the composition ratio of In toGa and Zn in each sputtering target is in parentheses. In FIG. 36, theamount of oxygen is excluded.

In FIG. 36, rhombuses indicate a homologous structure, and squaresindicate a spinel structure. The compositions of the sputtering targetsare plotted as white rhombuses and white squares, and the compositionsof the In—Ga—Zn oxide films formed are plotted as black rhombuses andblack squares.

FIG. 36 shows that the compositions of the In—Ga—Zn oxide films formedare different from the compositions of the sputtering targets.

Broken line L1 indicates a metal element ratio of Ga:Zn=2:1. Broken lineL2 indicates a composition ratio of trivalent metal ions (M³⁺) todivalent metal ions (M²⁺) of M³⁺:M²⁺=2:1.

FIG. 36 shows that the In—Ga—Zn oxide films having a spinel structureare formed in a region on the right side of broken line L1, i.e., aregion where the amount of Ga with respect to Zn increases, whereas theIn—Ga—Zn oxide films having a homologous structure are formed in aregion on the left side of broken line L1, i.e., a region where theamount of Zn with respect to Ga increases.

Table 2 shows the composition ratio of In to Ga and Zn, the atomic ratioof Zn to Ga (Zn/Ga), and the crystal structure of each of the In—Ga—Znoxide films included in the samples 1 to 9, and the composition ratio ofIn to Ga and Zn in the sputtering target used to form the film. Morespecifically, Table 2 shows composition ratios measured in In—Ga—Znoxide films formed by using a sputtering gas containing 33% of oxygen(in volume, and diluted with argon). Further, the crystal structures areindicated in cases of films formed by using a sputtering gas containing33% of oxygen (in volume, and diluted with argon) and for films formedby using a sputtering gas containing 100% of oxygen.

TABLE 2 In—Ga—Zn oxide film Target Composition Composition (O₂ = 33%)Crystal structure Sample In Ga Zn In Ga Zn Zn/Ga O₂ = 33% O₂ = 100%Sample 1 (132) 1 3 2 1 2.82 1.15 0.41 no peak spinel Sample 4 (162) 1 62 1 4.84 0.89 0.18 spinel spinel Sample 5 (164) 1 6 4 1 4.61 1.68 0.36no peak spinel Sample 8 (021) 0 2 1 0 2 1 0.50 spinel spinel Sample 2(134) 1 3 4 1 2.51 2.05 0.82 homologous homologous (spinel) Sample 3(136) 1 3 6 1 2.48 2.89 1.17 homologous homologous (spinel) Sample 6(168) 1 6 8 1 4.84 4.12 0.85 homologous homologous spinel Sample 7(1610) 1 6 10 1 5.20 5.20 1.00 homologous homologous (spinel) Sample 9(312) 3 1 2 3 1.01 1.29 1.28 homologous homologous

Table 2 suggests that the In—Ga—Zn oxide films formed using thesputtering targets containing Zn at a higher ratio than that of Ga havea homologous structure.

Table 2 also suggests that the In—Ga—Zn oxide films in which the ratioof Zn to Ga (Zn/Ga) is higher than 0.5 have a homologous structure.

The above results suggest that an In—Ga—Zn oxide film having ahomologous structure can be formed by using a sputtering targetcontaining Zn at a higher ratio than that of Ga.

<TEM>

FIGS. 37A to 37D show cross-sectional images (bright-field images)obtained by observing cross-sectional atomic arrangement of the samples2 and 3 using a transmission electron microscope (TEM). Here, regions inthe vicinity of surfaces of the In—Ga—Zn oxide films of the samples wereobserved at an acceleration voltage of 300 kV and at a magnification of8,000,000 times. In FIGS. 37A to 37D, the ratio of In to Ga and Zn inthe sputtering targets for the samples 2 and 3 is indicated inparentheses.

FIG. 37A is a cross-sectional image of the sample 2, and FIG. 37B is apartial enlarged cross-sectional image of FIG. 37A. FIG. 37C is across-sectional image of the sample 3, and FIG. 37D is a partialenlarged cross-sectional image of FIG. 37C.

As shown in FIGS. 37A to 37D, it can be seen that the samples 2 and 3have orderly atomic arrangement parallel to upper surfaces thereof. Thissuggests that the In—Ga—Zn oxide films of the samples 2 and 3 areCAAC-OS films.

<HAADF-STEM>

FIGS. 38A and 38B show cross-sectional images obtained by observingcross-sectional atomic arrangement of the samples 2 and 3 by high-angleannular dark field scanning transmission electron microscopy(HAADF-STEM). Here, the In—Ga—Zn oxide films of the samples wereobserved at an acceleration voltage of 200 kV and at a magnification of8,000,000 times.

FIG. 38A is a cross-sectional image obtained by observing the sample 2,and FIG. 38B is a cross-sectional image obtained by observing the sample3.

In a HAADF-STEM image, a contrast proportional to the square of anatomic number is obtained; therefore, a brighter dot indicates an atomwith a larger mass. In FIGS. 38A and 38B, bright dots indicated byarrows represent In atoms, and dark dots therebetween represent Ga or Znatoms. Note that it is difficult to distinguish Ga and Zn from eachother because they have roughly the same mass. In addition, oxygen atomshaving a small mass are not observed.

FIG. 38A shows that In, Ga, or Zn atoms are arranged in parallel layers.There are two or three layers of Ga or Zn atoms between layers of Inatoms indicated by the arrows.

FIG. 38B shows that In, Ga, or Zn atoms are arranged in parallel layers.There are three or four layers of Ga or Zn atoms between layers of Inatoms indicated by the arrows.

FIGS. 38A and 38B show that the In—Ga—Zn oxide films of the samples 2and 3 each have a homologous structure because there are periodic layersincluding In atoms, between which there are a plurality of layersincluding Ga or Zn atoms. In other words, an In—Ga—Zn oxide film havinga homologous structure can be formed by using a sputtering targetcontaining Zn at a higher ratio than that of Ga.

Example 2

In this example, films of In—Ga—Zn oxides having different compositionswere stacked using sputtering targets containing In—Ga—Zn oxides havingdifferent compositions. Band diagrams and crystal structures of thefilms formed will be described.

Here, the samples 1 to 4, 6, 7, and 10 described in Example 1 were used.Note that a silicon wafer was used as the substrate for each of thesamples 1 to 4, 6, 7, and 10.

Furthermore, a sample was formed by stacking over a silicon wafer theIn—Ga—Zn oxide film (20 nm thick) of the sample 1, the In—Ga—Zn oxidefilm (15 nm thick) of the sample 10, and the In—Ga—Zn oxide film (5 nmthick) of the sample 1 in this order. This sample is referred to as asample 11.

A sample was formed by stacking over a silicon wafer the In—Ga—Zn oxidefilm (20 nm thick) of the sample 2, the In—Ga—Zn oxide film (15 nmthick) of the sample 10, and the In—Ga—Zn oxide film (10 nm thick) ofthe sample 2 in this order. This sample is referred to as a sample 12.

A sample was formed by stacking over a silicon wafer the In—Ga—Zn oxidefilm (20 nm thick) of the sample 3, the In—Ga—Zn oxide film (10 nmthick) of the sample 10, and the In—Ga—Zn oxide film (15 nm thick) ofthe sample 3 in this order. This sample is referred to as a sample 13.

<Band Diagram>

An energy difference between the conduction band bottom E_(c) and thevalence band top E_(v), i.e., the energy gap E_(g), of the In—Ga—Znoxide film of each of the samples 1 to 4, 6, 7, and 10 formed in Example1 was measured using a spectroscopic ellipsometer. Furthermore, anenergy difference between the vacuum level E_(vac), and the valence bandtop E_(v), i.e., the ionization potential Ip, was measured byultraviolet photoelectron spectroscopy (UPS). Then, an energy differencebetween the vacuum level E_(vac), and the conduction band bottom E_(c),i.e., the electron affinity χ, was calculated by calculating adifference between the ionization potential Ip and the energy gap E_(g).Band diagrams thus obtained are shown in FIG. 39.

As shown in FIG. 39, the samples 1 to 4, 6, and 7 have smaller electronaffinity χ than that of the sample 10, and the energy differences aregreater than or equal to 0.2 eV and less than or equal to 0.4 eV.

Next, band diagrams of the samples 12 and 13 are described withreference to FIGS. 40A and 40B.

FIG. 40A shows a band diagram obtained by measuring the ionizationpotential Ip and the energy gap E_(g) of the sample 12, and FIG. 40Bshows a band diagram obtained by measuring the ionization potential Ipand the energy gap E_(g) of the sample 13. In each of FIGS. 40A and 40B,the horizontal axis represents sputtering time, and the vertical axisrepresents the energy difference between the valence band top E_(v) andthe vacuum level E_(vac) (E_(v)−E_(vac)) and the energy differencebetween the conduction band bottom E_(c) and the vacuum level E_(vac)(E_(c)−E_(vac)).

Here, the energy difference between the vacuum level E_(vac) and thevalence band top E_(v), i.e., the ionization potential Ip, of theIn—Ga—Zn oxide film of each of the samples 12 and 13 was plotted whichwas measured by ultraviolet photoelectron spectroscopy while a surfaceof the In—Ga—Zn oxide film was sputtered with argon ions. Furthermore,the energy difference between the conduction band bottom E_(c) and thevalence band top E_(v), i.e., the energy gap E_(g), was measured using aspectroscopic ellipsometer. Next, the energy difference between thevacuum level E_(vac) and the conduction band bottom E_(c), the electronaffinity χ, was calculated by calculating the difference between theionization potential Ip and the energy gap E_(g). Then, the valence bandtop E_(v) and the conduction band bottom E_(c) were plotted.

FIG. 40A shows that the energy difference between the conduction bandbottom E_(c) of the In—Ga—Zn oxide film of the sample 2 and that of theIn—Ga—Zn oxide film of the sample 10 is 0.2 eV. It can be seen that theIn—Ga—Zn oxide film of the sample 10 serves as a well when providedbetween the In—Ga—Zn oxide films of the sample 2.

FIG. 40B shows that the energy difference between the conduction bandbottom E_(c) of the In—Ga—Zn oxide film of the sample 3 and that of theIn—Ga—Zn oxide film of the sample 10 is 0.2 eV. It can be seen that theIn—Ga—Zn oxide film of the sample 10 serves as a well when providedbetween the In—Ga—Zn oxide films of the sample 3.

<TEM>

FIGS. 41 to 43 show cross-sectional images (bright-field images)obtained by observing cross-sectional atomic arrangement of the samples11 to 13 using a transmission electron microscope (TEM). Here, theIn—Ga—Zn oxide films of the samples were observed at an accelerationvoltage of 300 kV and at magnifications of 2,000,000 times and 4,000,000times.

FIG. 41 shows cross-sectional images of the samples 11 to 13 observed ata magnification of 2,000,000 times. FIG. 42 shows cross-sectional imagesof the samples 11 to 13 observed at a magnification of 4,000,000 times.FIG. 43 shows partial enlarged cross-sectional images of FIG. 42. InFIGS. 41 to 43, the In—Ga—Zn oxide film stacked are denoted by S1, S2,and S3 sequentially from the substrate side. In FIG. 43, white brokenlines indicate the vicinity of the interface between S1 and S2.

As shown in FIG. 41, in the samples 12 and 13, S1 to S3 formed usingdifferent sputtering targets have uniform crystallinity.

As shown in FIG. 42, in the sample 11, crystallinity is not observed inS1, and crystallinity is observed only in S2. On the other hand, in thesamples 12 and 13, crystallinity is observed in S1 to S3.

As shown in FIG. 43, it can be seen that the In—Ga—Zn oxide films of thesamples 12 and 13 have orderly atomic arrangement parallel to the uppersurfaces. At the interface between S1 and S2, orderly parallel atomicarrangement can be seen. This suggests that S1 and S2 of each of thesamples 12 and 13 are CAAC-OS films, and that there is little crystaldistortion at the interface between S1 and S2.

An In—Ga—Zn oxide film having a homologous structure can be formed byusing a sputtering target containing Zn at a higher ratio than that ofGa. Furthermore, a CAAC-OS film can be formed by using a sputteringtarget containing Zn at a higher ratio than that of Ga. An In—Ga—Znoxide film including a plurality of layers stacked using sputteringtargets having different compositions has a uniform crystal structurethroughout the film and has little crystal distortion at the interfacebetween the layers.

Therefore, the amount of defects in the In—Ga—Zn oxide film formed inthis example can be reduced, changes in electrical characteristics of asemiconductor device formed using the In—Ga—Zn oxide film can bereduced, and the reliability thereof can be improved.

This application is based on Japanese Patent Application serial no.2013-038402 filed with Japan Patent Office on Feb. 28, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a sputtering target,comprising the steps of: forming a polycrystalline In-M-Zn oxide powderfrom a homologous compound of an In-M-Zn oxide, M representing a metalselected from the group consisting of aluminum, titanium, gallium,yttrium, zirconium, lanthanum, cesium, neodymium, and hafnium; forming amixture by mixing the polycrystalline In-M-Zn oxide powder and a zincoxide powder; forming a compact by compacting the mixture; and sinteringthe compact.
 2. The method for manufacturing a sputtering targetaccording to claim 1, wherein an atomic ratio of zinc in the sputteringtarget is higher than an atomic ratio of M in the sputtering target. 3.The method for manufacturing a sputtering target according to claim 1,wherein M represents gallium.
 4. A method for manufacturing a sputteringtarget, comprising the steps of: forming a polycrystalline In-M-Zn oxidepowder, M representing a metal selected from the group consisting ofaluminum, titanium, gallium, yttrium, zirconium, lanthanum, cesium,neodymium, and hafnium, by mixing, sintering, and grinding indium oxide,an oxide of the metal, and zinc oxide; forming a mixture by mixing thepolycrystalline In-M-Zn oxide powder and a zinc oxide powder; forming acompact by compacting the mixture; and sintering the compact.
 5. Themethod for manufacturing a sputtering target according to claim 4,wherein an atomic ratio of zinc in the sputtering target is higher thanan atomic ratio of M in the sputtering target.
 6. The method formanufacturing a sputtering target according to claim 4, wherein thepolycrystalline In-M-Zn oxide powder is a homologous compound.
 7. Themethod for manufacturing a sputtering target according to claim 4,wherein M represents gallium.